Lecture 8 Notes
A Short Intro to Botany
| I. Basic Plant Anatomy
A. Basic Tissue Types 1. Ground - becomes pith and cortical cells 2. Vascular - becomes xylem and phloem and support cells (fibers, sclereids) 3. Dermal - becomes epidermis and endodermis 4. Meristems - meristems are sites of continual cell division. Here cells mature and differentiate into various other cells. All plants grow from apical or lateral (on the side) meristem regions. Shoots have an apical meristem, and so do roots. Leaves have them either at their base (in grasses) or on the sides of the blades in other types of plants. B. Basic Cell types 1. Ground Tissue a. Parenchyma - basic undifferentiated cell - has the potential to become other cell types. Has all organelles, thin primary cell wall. Click here to see examples: http://www.biologie.uni-hamburg.de/b-online/e05/05d.htm#02 b. Collenchyma - similar to parenchyma cell, but with thickened cell walls. Often used for support. Strands in celery are collenchyma cells. Click here for more: http://www.biologie.uni-hamburg.de/b-online/e06/06.htm#collen c. Sclerenchyma - cells have secondary cell walls in addition to primary cell wall. Provides more support and strength to cells. Used to support tissues such as xylem and phloem. Click here for a picture: http://www.biologie.uni-hamburg.de/b-online/e06/sklerenc.htm Two types of sclerenchyma cells: i. fibers - long slender cells ii. sclereids - often branched, spherical shaped. Grittiness in pears is due to these cells. Gives shells of nuts their hardness. 2. Dermal Cells Click here to see various types of epidermal cells a. Guard cells - found mainly in leaf epidermis (but some epiphytic orchids have them on their roots!). Used to regulate uptake of carbon dioxide and loss of water from plant. The guard cell complex is known as a stoma (stomata is plural). Click here to see guard cells: http://www.biologie.uni-hamburg.de/b-online/e05/r05.htm b. Trichomes - hair-like outgrowths of epidermis. Deters herbivores, shields plant from excess radiation, may act to trap heat next to leaf, etc. c. Root hairs - hair-like extensions of root epidermis. Important in the uptake of water and nutrients. May be thousands of feet of root hairs on a single plant, if not miles of them! Click here for more on root hairs. 3. Vascular Cells a. Xylem - conducting cells dead at maturity. All that remains is cell wall. All conducting cells have secondary cell walls for increased strength. Two kinds of cells involved: i. tracheids - evolutionarily the oldest type of conducting cell. Long and narrow, with end walls. Walls perforated with pits, which allow water to move from cell to cell. Found in almost all vascular plants. Click here to see tracheids: http://www.biologie.uni-hamburg.de/b-online/e06/tracheid.htm Click here to see pits: http://www.biologie.uni-hamburg.de/b-online/e06/abieshof.htm ii. vessel elements - more recently evolved. Only found in angiosperms. Are shorter and wider than tracheids. End walls only found occassionally. A group of vessel elements bounded by cells with end walls is known as a vessel. Vessels may be as small as 2-3 cells in a row, or up to 10 meters of more in length (red oaks, for example), involving thousands of cells in a row. Depends on the species involved. Click here to see vessels: http://www.biologie.uni-hamburg.de/b-online/e06/mazerirt.htm http://www.biologie.uni-hamburg.de/b-online/e06/calamus.htm http://www.biologie.uni-hamburg.de/b-online/e06/rastrach.htm iii. fibers and sclereids - living cells that provide support for the xylem. b. Phloem - conducting cells alive at maturity. Principal food conducting tissue in plants. Click here for more on phloem. Two major kinds of cells involved: i. sieve tube cells - alive at maturity, but with no organelles. No nucleus or internal organelles. Just cytoplasm. End walls perforated with small holes known as sieve pores. The entire end wall is known as a sieve plate. These cells conduct water and photosynthates throughout the plant. Cells are stacked in long tubes, like vessels elements. Click here to see sieve tube cells. ii. companion cell - alive at maturity. Derived from same mother cell as a sieve tube cell. Has normal set of organelles, including nucleus, mitochondria, etc. iii. fibers and sclereids - again, for support. C. Basic Organ Types 1. Shoots - Technically, shoots are tissues to which cotyledons are attached. Cotyledons are first set of leaves after germinating from seed. a. stems - where leaves are attached is called a node. Inbetween, the area is called an internode. b. axillary buds - develop above petiole at point of attachment to stem - are usually dormant, but can sprout and make new leaves if old one is injured. c. pith - innermost portion of a stem, usually surrounded by undifferentiated cells called cortex. d. vascular tissues - phloem is to the outside, xylem to the inside. Each group of vascular tissues is known as a vascular bundle. In dicots, bundles are scattered about the periphery of the stem. In monocots, they are scattered throughout the stem cross-section. Click here for a detailed discussion of vascular bundles. Click here for vascular bundles in dicots. Click here for vascular bundles in monocots. 2. Leaves - light capturing organs. In some plants, such as cacti and euphorbias, leaves have been lost, or drop during droughts, and stem does photosynthesis. Most cells in leaf are parenchyma cells except vascular cells. Click here for more on leaves. a. leaves grow from marginal meristems (at edges of leaves). In grasses, the meristem is basal, that is, at base of leaf. Thus, tip of leaf is oldest part of leaf. This is why if you mow grass it can grow back! Some cross sections of leaves if you click here. i. leaf blade - flattened portion that does most of the photosynthesis and transpiration 1. three major cell types in leaf blade a. epidermis - on top and bottom of leaf, contains stomata b. palisade mesophyll - upper layer of photosynthetic cells, just below upper epidermis. In sun leaves there may be two layers, in shade leaves just one. Cells shaped like slender columns, tightly packed together, few airspaces. c. spongy mesophyll - lower layer of photosynthetic cells, below the palisade cell layer. Cells are irregularly shaped, widely spaced with lots of airspaces inbetween. ii. petiole - stem-like structure that attaches leaf to stem. iii. stipules - small, leaf-like structures on either side of petiole where it attaches to stem. 3. Roots - belowground (usually, but in some plants, roots can be aboveground). Click here for more detail on roots. a. From the outside in you encounter the following layers: i. epidermis with root hairs, and root cap covering root tip ii. cortex iii. endodermis with Casparian strip iv. pericycle v. stele with xylem and phloem. The stele is the innermost area of the root, and contains the xylem and phloem. b. Details of each layer i. epidermis - outer layer, where root hairs help in absorption of water and nutrients. Root cap is a set of cells that protects the root apical meristem. ii. cortex - large, undifferentiated cells that separate the stele from the epidermis. Mainly for support and movement of water and nutrients to the endodermis. Since water moves along the path of least resistance, most of it flows along the cortical cell walls rather than passing through cortical cell cytoplasm, which would really slow the flow down. iii. endodermis - inner layer of cells that contains a suberized layer called the Casparian Strip. Casparian strip makes the cell walls impermeable to water flow, so water and materials coming into the root through the cortex have to pass across the cell membrances of the endodermis. It is at this layer in the cell that the root regulates the uptake of nutrients into the stele. iv. pericycle - this layer of meristematic cells can give rise to lateral roots. The new roots actually grow horizontally through the endodermis, cortex, and outer epidermis until they break through into the soil. v. stele - xylem and phloem located here. Xylem is central-most in the root cross-section. Phloem tends to be nestled in the arcs between the arms xylem. 4. We won't be discussing woody plant growth this time around - not enough time. Maybe in a later lecture. |
A truly great website to help you learn the terminology of Botany can be found at: http://www.biologie.uni-hamburg.de/b-online/ereg/eindexfr.htm
Additional information in an online botany text is available
at:
http://www.biologie.uni-hamburg.de/b-online/e00/contents.htm