TEXTURE 139 



3. Tubers. Tubers are thick, fleshy underground stems 

 like the potato. Like the bulbs they serve chiefly as storage 

 organs to provide food for the year's growth. 



Other Modified Stems. Although in the cactus the stem 

 is leafless it manufactures food because of the presence of 

 chlorophyll. In asparagus the leaves are reduced to scales 

 but the stem divides into small sections which are green and 

 leaflike and which function as leaves. In hawthorn the stem 

 is modified to thorns which in some species become branched 

 like true stems. In the grape many of the lateral stems 

 become modified to tendrils which enable the plant to climb 

 frequently to great heights. In Virginia creeper the ends 

 of the tendril-like stems form into disks which adhere to 

 the bark of trees and enable the plant to climb ; as the wood- 

 bine grows older, however, aerial roots develop and penetrate 

 the bark, and the disc-like stems disappear. 



Direction of Growth. Plants, like lobelia, that assume a 

 vertical position are erect', when, like the branches of 

 viburnum, they grow obliquely upward they are ascending; 

 when, like veronica, the plant grows along the ground but 

 with the growing tip directed obliquely upward, it is decum- 

 bent', when, like partridge vine, the entire stem lies flat upon 

 the ground it is procumbent. When, like wild strawberry, 

 the stem grows along the ground and develops roots at the 

 joints, it is repent', when, like mandrake, the stem grows 

 under the ground it is subterranean. 



Duration. Stems, such as herbs, that live for one year 

 are annual, those that live for two years are biennial while 

 those that live for several years, like the stems of woody 

 plants, are perennial. 



Texture. Stems are fleshy when the parenchymatic tissue 

 exceeds in amount the other tissues, fibrous when they have 

 considerable mechanical tissue such as bast and woody fibers 



