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. 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. 

 95 2 > 953)- When an 

 apogeotropic stem is 



placed horizontally, the FIG. 954. A plant of Euphorbia maculata, illus- 



growing tip SOOn becomes trating the prostrate habit; note also that, although 



erect, but usually the older thC &**** ? ecus fe, the leaves are in one plane 

 * 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 differential 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 life, 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 and in many water plants), while in other water plants (as in 

 Potamogeton) the reproductive stems are more rigidly erect than are the 

 vegetative stems. 



Lateral branches. The most important exception to erectness in 

 aerial stems is in the lateral branches, which grow in various directions, 

 thus resembling the diverse directions of lateral roots. The resulting 

 plant contour usually is symmetrical, especially in those conifers whose 

 trunks are excurrent (i.e. extending to the summit), and whose outline 

 is approximately an elongated cone (fig. 955). The cause of directional 



