Chapter 33

1. Plant organs, such as leaves and roots, have structures that allow maximal acquisition of gases and nutrients. Also, plants can grow throughout their lifetimes, enabling them to respond to environmental cues. A plant can redirect its growth to exploit opportunities in its immediate environment.

2. Plants grow throughout their lifetimes; animal organs often stop growing at adulthood. Plant cells are totipotent; in animals, only the zygote and perhaps the earliest embryonic cells are totipotent. Plant cells divide unevenly in cytokinesis to orient organ development; in animal development, cell movements are prominent.

3. Apical–basal patterns develop in plants by unequal cell division leading to differentiation between daughter cells destined for the tip, or apex, of the plant and those destined for its base in the early embryo. Radial patterns develop by cell differentiation, whereby the embryo is first a sphere and then a cylinder.

4. Without apical–basal polarity, there will be just a massive globular embryo, and root and shoot apices will fail to develop. So these organs will could arise at any place in the developing plant, or not arise at all.

1. Tissue	Structure	Function
   Parenchyma	Thin-walled, large vacuoles	Photosynthesis in leaves, storage in roots
   Collenchyma	Elongated, thick-walled at corners	Flexible support in petioles and stems
   Sclerenchyma	Thick-walled, bundles	Support of stems, occurs in fruits

2. Collenchyma has primary walls at the corners of cells and is somewhat flexible; sclerenchyma has thick, less-flexible secondary walls.

3. When tracheids die, pits in their walls remain and allow water to flow between the cells. Vessel elements are long tubes, laid end-to-end, that die and form a continuous tube.

1. An apical meristem has stem cells that continuously divide. Some of the daughter cells form new organs (e.g., leaves), but others remain to divide further as part of the undifferentiated pool.

2. Root and shoot apical meristems give rise to primary meristems, which are responsible for primary growth of plants and form all plant tissues. There are three primary meristems: (1) protoderm forms dermal tissues; (2) ground meristem forms ground tissues; and (3) procambium forms vascular tissues.

3. The root apical meristem forms the root cap and root primary meristem. The latter forms the dermal, ground, and vascular tissues of the root. The root grows by cell expansion in a region just above the root apical meristem.

4. Primary: apical meristem; secondary: lateral meristem (vascular cambium) and lateral meristem (cork cambium)

   Primary: Xylem, phloem, parenchyma, dermal tissue; secondary: secondary xylem and phloem, cork

   Primary: growth in length; secondary: growth in thickness

1. Seeds of wild relatives of crop plants have genes that may have been selected against during plant domestication. These genes might encode phenotypes that adapt the plants to their wild environment but were not valuable to farmers at the time of domestication. But the environment can change, and the genes that were selected against can again become valuable. An example might be a changing climate where roots that grow deep would be advantageous. The cultivated crop may lack genes for this phenotype, but genes for root growth might still be present in a wild relative.

1. CG appears to be highest in the upper leaves and shoot, where it is made. The lower leaves and stems contain less CG, indicating that CG is transported there on the way to other plant organs.

2. The fifth leaf accumulated CG, which was not transported down the plant vascular system because of girdling. There was some residual accumulation in the sixth leaf that had probably arrived before the girdling. The stem below the sixth leaf had very low CG, indicating the transport was blocked.

1. The cells are incubated in radioactive thymidine for 2 hours, during which dividing cells take up the radioactive substance and incorporate it into their DNA. After the cells are removed and placed in nonradioactive thymidine, dividing cells take up the nonradioactive substance. Thus cells dividing more than 2 hours after the start of the experiment will not be labeled. As cells formed in the first 2 hours begin to elongate and mature, the cells are pushed up the root by the new cells being made. As cells move from the zone of division to the zone of elongation, the number of labeled cells in the zone of division decreases. As cells move from the zone of elongation to maturation, the number of labeled cells in the zone of elongation decreases.

2. The number of labeled cells increases first in the zone of division, then in the zone of elongation, and finally in the zone of maturation. This indicates that root cells are added at the bottom by cell division. The cells then grow and elongate the root, then finally differentiate into the various tissue types that make up the mature root. Many cells are present in the zone of division, as the initials divide. It takes time for these new daughter cells to grow and move up the root, as they are pushed by newly formed cells below them.

3. Initials in specific locations grow up from their original locations in the meristem, with the initials forming epidermal cells located on the outside, those forming the cortex inside this, and those forming transport tissues nearest the center. These regions form the protoderm, ground tissue, and procambium, which are the precursors that differentiate into all types of root tissue. As they grow upward without migrating, the tissues naturally orient into a cylinder with several layers of cells.

4. Cells in the QC divide very slowly; in this experiment, a single cell was dividing during the first 2 hours (when labeled thymidine was present), but no more division occurred after that time. There could very well have been no division during these 2 hours, as previous experiments show that corn QC cells divide only once every 170 hours. Also, the QC contains very few cells (the exact number varies by species). This indicates that it contributes only a few cells to the growing root. These few cells might replenish initial cells lost through damage or aging, without causing excess growth of root tissue. However, this experiment does not show how (or whether) cells move from the QC to the initials.