AERIAL STEMS 173 



The erect stem is the common type of aerial stem, and is best 

 adapted for leaf display. Having the erect position, it can 

 branch and display leaves on all sides, and by elongation can lift 

 its leaves above the shade of other plants. 



Erect stems show a striking contrast to primary roots in the 

 way they respond to gravity and light. While the main axis of 

 primary roots is positively geotropic and negatively heliotropic, 

 the trunk of erect stems behaves in the opposite way, thus grow- 

 ing away from the center of the earth and toward the light. But, 

 like lateral roots, the branches of the shoot tend to take a hori- 

 zontal or plagiotropic position, in which they appear indifferent to 

 both light and gravity. However, this indifference to gravity 

 and light on the part of the branches of the shoot seems to depend 

 upon influences exerted by other parts of the stem; for, if the 

 upper part of the shoot is removed, then the horizontal branches 

 remaining show a strong tendency to become erect. 



Erect stems, being wholly self-supporting and much exposed 

 to winds, surpass other stems in amount of strengthening tissue 

 developed. From this type of stems where woody tissue reaches 

 its maximum development, as in trees, we obtain timber. How- 

 ever, erect stems are not always sufficiently strong to endure the 

 strains to which they are exposed, as is well known in case of 

 grains where the so-called " lodging " often occurs. 



In size erect stems surpass all others. The most remarkable 

 erect stems are those of the giant Sequoias, one of which is 

 shown in Figure 149. These giant trees, which are natives of 

 the western mountains, may attain a height of 400 feet, a cir- 

 cumference of more than 100 feet, and live to be more than 4000 

 years old. 



The prostrate stems of Pumpkins, Melons, Squashes, Cucumbers, 

 Strawberries, and Sweet Potatoes are well known to the student. 

 They are not strong enough to maintain an erect position, and 

 lie stretched upon the ground. Prostrate stems are common 

 among such weeds as the Five-fingers and Spurges. Some plants, 

 as Crab Grass and Buffalo Grass illustrate, have both erect and 

 prostrate stems. In this case the erect stems arise as branches 

 from the prostrate ones. 



The prostrate position is not a good one for leaf display; for 

 leaves can be displayed only on the upper side and not all around 

 as in erect stems. Prostrate stems are also not able to escape 



