Topics: lawn care , the science behind holganix. Our Team. Agriculture Training. Marketing Tools For Landscapers. Call: By Kaitlyn Ersek on Sep 14, AM Plants can be separated into two distinct categories: monocots and dicots. Monocot vs. Dicot Monocots differ from dicots in four distinct structural features: leaves, stems, roots and flowers. Roots: Fibrous vs. Stems: Arranging the vascular tissue As the monocots develop, the stem arranges the vascular tissue the circulatory system of the plant sporadically.
Leaves: Parallel veins vs. Flowers: How many flower petals does your plant have? What Does This Mean for You? Works Cited Phelan, Jay. Related posts. Subscribe Here! Many tropical plant species have exceptionally broad leaves to maximize the capture of sunlight. Other species are epiphytes: plants that grow on other plants that serve as a physical support. Such plants are able to grow high up in the canopy atop the branches of other trees, where sunlight is more plentiful. Epiphytes live on rain and minerals collected in the branches and leaves of the supporting plant.
Bromeliads members of the pineapple family , ferns, and orchids are examples of tropical epiphytes Figure. Many epiphytes have specialized tissues that enable them to efficiently capture and store water.
Some plants have special adaptations that help them to survive in nutrient-poor environments. Carnivorous plants, such as the Venus flytrap and the pitcher plant Figure , grow in bogs where the soil is low in nitrogen.
In these plants, leaves are modified to capture insects. The insect-capturing leaves may have evolved to provide these plants with a supplementary source of much-needed nitrogen.
Many swamp plants have adaptations that enable them to thrive in wet areas, where their roots grow submerged underwater. In these aquatic areas, the soil is unstable and little oxygen is available to reach the roots. Trees such as mangroves Rhizophora sp. Some species of mangroves, as well as cypress trees, have pneumatophores: upward-growing roots containing pores and pockets of tissue specialized for gas exchange.
Wild rice is an aquatic plant with large air spaces in the root cortex. The air-filled tissue—called aerenchyma—provides a path for oxygen to diffuse down to the root tips, which are embedded in oxygen-poor bottom sediments. Leaves are the main site of photosynthesis. A typical leaf consists of a lamina the broad part of the leaf, also called the blade and a petiole the stalk that attaches the leaf to a stem. The arrangement of leaves on a stem, known as phyllotaxy, enables maximum exposure to sunlight.
Each plant species has a characteristic leaf arrangement and form. The pattern of leaf arrangement may be alternate, opposite, or spiral, while leaf form may be simple or compound. Leaf tissue consists of the epidermis, which forms the outermost cell layer, and mesophyll and vascular tissue, which make up the inner portion of the leaf. In some plant species, leaf form is modified to form structures such as tendrils, spines, bud scales, and needles.
Monocots have leaves with parallel venation, and dicots have leaves with reticulate, net-like venation. Describe an example of a plant with leaves that are adapted to cold temperatures. Conifers such as spruce, fir, and pine have needle-shaped leaves with sunken stomata, helping to reduce water loss. Skip to content Plant Form and Physiology. Learning Objectives By the end of this section, you will be able to do the following: Identify the parts of a typical leaf Describe the internal structure and function of a leaf Compare and contrast simple leaves and compound leaves List and describe examples of modified leaves.
Structure of a Typical Leaf Each leaf typically has a leaf blade called the lamina , which is also the widest part of the leaf. Deceptively simple in appearance, a leaf is a highly efficient structure. The netlike venation in this b linden Tilia cordata leaf distinguishes it as a dicot. The c Ginkgo biloba tree has dichotomous venation. Leaf Arrangement The arrangement of leaves on a stem is known as phyllotaxy. Leaf Form Leaves may be simple or compound Figure. Leaves may be simple or compound.
In simple leaves, the lamina is continuous. The a banana plant Musa sp. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In b palmately compound leaves, such as those of the horse chestnut Aesculus hippocastanum , the leaflets branch from the petiole.
In c pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory Carya floridana. The d honey locust has double compound leaves, in which leaflets branch from the veins.
Leaf Structure and Function The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Visualized at x with a scanning electron microscope, several stomata are clearly visible on a the surface of this sumac Rhus glabra leaf. At 5,x magnification, the guard cells of b a single stoma from lyre-leaved sand cress Arabidopsis lyrata have the appearance of lips that surround the opening.
Development usually proceeds basipetally for the higher orders of vasculature so that the minor veins are present at the leaf apex while the secondary veins are still forming near the petiole. Most grasses initiate the midvein and the secondary vasculature from the disk of leaf insertion and not from the existing stem provasculature.
The midvein elongates acropetally toward the leaf tip. Only after the commencement of acropetal growth does the midvein also commence development toward the stem vascular strand, forming the leaf trace. Large longitudinal veins parallel to the midvein also form in this manner, arising from the disk of leaf.
Next, the intermediate provascular strands initiate in the leaf apex and development extends basipetally to connect to the existing major longitudinal veins that have developed in the leaf. Only some of the basipetally extending intermediary veins develop through the leaf sheath to connect with the stem vascular bundle through a leaf trace.
Finally, small longitudinal veins develop commencing near the apex of the leaf and extending basipetally to connect to the higher order veins at about the leaf sheath-leaf junction.
The transverse veins also develop starting near the apex of the leaf and extending basipetally to provide the leaf with a reticulate network of vasculature. The reader is cautioned that the rather novel developmental sequence with the midvein and large longitudinal veins commencing development in the disk of leaf attachment without any apparent connection with the provascular trace of the stem may be just that Our limitations identifying provascular tissue prior to its advanced development may prevent the recognition of an existing leaf trace until after the midvein in the disk of leaf attachment has commenced differentiation.
Vein spacing:. The most obvious manifestation of uniform vein spacing is seen in the leaf blade of grasses that have a constant longitudinal vein number per unit lateral blade width. In the dicots, even if the polygonal shape of the ultimate leaf areoles are remarkably diverse, the occurrence of branch points from veins and veinlets is remarkably uniform. Constant branch points from veins.
Models for the Regulation of Vascular Pattern Formation:. Any hypothesis that attempts to describe vascular pattern formation must account for three divergent phenomenon; 1 the acropetally oriented formation of major veins in developing dicot leaves; 2 the formation of isolated, parallel provascular tissue in expanding grass leaves and; 3 the simultaneous formation of minor veins and transverse veinlets in both dicots and monocots over large areas of the leaf.
The two best models only imperfectly describe how vascular patterning might arise. Model 1: Canalization of signal flow:. All cells start out being equivalent transporters of auxin, a hormone implicated in the induction of vascular differentiation. Stochastically, some cells transport more auxin, and this greater contact with auxin enhances their ability to transport more of it, creating a positive feedback loop.
The greater auxin flux through these cells eventually induces them to become provascular cells and drains surrounding cells of auxin, inhibiting them from also becoming provascular tissue. Additionally, the auxin is passed basipetally to the next cell in the file which now accrues its own auxin plus all the auxin from the cell above it, converting it to provascular tissue.
This hypothesis can account for the type of vascular development seen in dicot leaves but cannot account for how the provascular tissue in monocots appears to develop, nor the simultaneous development of minor veins throughout a large section of the leaf.
Model 2: Diffusion-reaction prepattern:. This hypothesis can account for the parallel, and simultaneous, formation of longitudinal veins in monocot leaves. Additionally, it can account for the intercalary growth of new veins between older veins as the leaf blade expands because the concentration of the inhibitory morphogen would be depleted the further apart the two veins moved until it was no longer sufficient to inhibit a new wave of provascular tissue formation.
This would also tend to promote very uniform spacing between veins, their simultaneous formation, and produce patches where no venation would occur…areoles. Current theory is that, in order to explain much of what we know about vascular tissue differentiation, we will have to come up with a joining of aspects of the two models above. Comparison among leaf, stem, and root vasculature:.
The most noticeable difference between the vasculature of the root, stem and leaf is in the symmetry of the organs. In roots, the vasculature forms a central pith-filled, or solid cylinder that is radially symmetrical and whose organization is not greatly influenced by the occurrence of peripheral organs.
In the stem, the vasculature is organized into radially symmetrical, sympodial bundles whose organization is in direct relation to shoot phyllotaxis i.
At each dicot node, at least three vascular bundles diverge from separate sympodial bundles to serve the leaf at that node. The remainder of the sympodial bundle continues through the next internode.
The divergent vascular bundles, so called leaf traces, arising as they do from independent sympodial bundles, provide redundancy in the water supply of the leaf. The architecture of the sympodial bundle seldom varies having the phloem situated to the outside of the xylem. The position of the xylem towards the adaxial upper portion of the typically dorsiventral leaf and the phloem towards the abaxial region reflects the architecture in the stem from whence the leaf trace originates.
Vegetative development: Phloem and Xylem:. Let us examine two components of plant vasculature, phloem and xylem. One fundamental difference in how animals and plants transport assimilate is that while both use vessels made from cells, the smallest of these vessels in animals is comprised of cells but does not have majority of the transport passing through the cells themselves but rather through vessels formed by these cells capillaries. In plants of course, transport is through the phloem and xylem cells themselves.
Vascular differentiation in plants is difficult to study due to the position of the vascular elements, buried within the plant body, the relatively few cells comprising the vasculature, and the even fewer cells undergoing differentiation at any one time relative to the number of differentiated vascular cells. Much of what is known about vascular tissue differentiation at the molecular level has been acquired in the past two decades with the advent of an inducible cell culture system Zinnia elegans for xylem providing quantities of more-or-less synchronized cells following the same developmental pathway.
Tissue culture systems have similarly been adopted for studies of phloem differentiation. In the seedling there develops primary vasculature comprised of protophloem and protoxylem which are quickly crushed and torn apart as the seedling elongates. They serve to transport water and nutrients during the early stages of establishment and are quickly replaced by the metaphloem and metaxylem. This vasculature is more long lived, developing after most of the cells comprising the seedling have finished elongating.
Additional files of cells are added to the existing metaphloem and metaxylem as the meristems produce them. While the protophloem has no companion cells, the metaphloem does, enabling it to survive for considerably longer periods. Companion cells are associated with mature sieve elements and are thought to be necessary for sieve element function and survival. The role of the companion cell in phloem loading sieve element function will be dealt with below. Correlative evidence supporting the conjecture that companion cells are responsible for sieve element survival arises from studies of protophloem elements in developing leaves and stems and which lack companion cells.
These protophloem elements are short-lived after they have differentiated and are replaced later in development by metaphloem sieve elements which have companion cells and which live much longer years in the case of palms.
The companion cells must produce the proteins for the mature sieve elements they serve because the mature elements are without ribosomes. Without a mechanism for producing proteins de novo the life span of any cell would be short indeed. Even the P-protein, necessary for avoiding catastrophic failure and possible infection of large portions of the phloem system and surrounding tissue upon injury of an element, is manufactured in the companion cells and transported to the mature element.
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