. A textbook of botany for colleges and universities ... Botany. STEMS 647 the conifers and dicotyls. Thus it appears to be associated with those groups that exhibit considerable growth in diameter from year to year, and it can be recognized readily that the enormous spread of a dicotyl tree would be quite impossible but for such diametral enlargement of the trunk. However, the connection between diametral enlargement and branching is not absolute, as is indicated by the cycads, which rarely branch, though increasing in stem diameter. Some monocotyls and some extinct pteridophytes exhibit bran
. A textbook of botany for colleges and universities ... Botany. STEMS 647 the conifers and dicotyls. Thus it appears to be associated with those groups that exhibit considerable growth in diameter from year to year, and it can be recognized readily that the enormous spread of a dicotyl tree would be quite impossible but for such diametral enlargement of the trunk. However, the connection between diametral enlargement and branching is not absolute, as is indicated by the cycads, which rarely branch, though increasing in stem diameter. Some monocotyls and some extinct pteridophytes exhibit branching or increase in stem diameter or sometimes both combined. Stem erectness. â Most aerial stems tend to grow erect, being pro- phototropic and apogeotropic. In a dark chamber, erectness is due solely to apogeotropism, but in ordinary habitats light and gravity co- operate in determining erectness, the light influence being the stronger, as is well shown when plants are exposed to one- sided illumination (figs. 952, 953)- When an apogeotropic stem is placed horizontally, the -Fig. 954. â A plant of Euphorbia maculata, illus- growine tip soon becomes trating the prostrate habit; note also that, although . , . II ii ij the phyllotaxy is decussate, the leaves are in one plane erect, but usually the older â \. ,u \. â ^- i »i. u â * 1 â . ' â¢' owing to the twisting of the horizontal stems. parts of the stem remain horizontal; however, in certain grasses (as in the cereals) the whole stem once more becomes erect through difierential growth in the lower nodes. Most subterranean stems and some aerial stems, particu- larly those that are prostrate or running, show little tendency toward erectness (fig. 954). Many of the latter are erect when young and have erect tips through Ufe, suggesting that horizontality in such cases may be due, in part at least, to the lack of sufficient mechanical tissue to permit of erectness. Some fruiting stems grow downward (as in the peanut a
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