TEXTBOOK OF BOTANY 



grasses is due to the interlacing of their underground stems 

 and roots. Certain sedges and grasses have been largely 

 planted on the dykes in Holland, and on sand dunes on the 

 French coast which formerly drifted and were blown about, 

 covering forests, houses, and even villages. The under- 

 ground stems, branches, and roots form a tangle that binds 

 the loose sand into a firm soil, in which trees may then be 

 planted. The underground stems that we have been con- 

 sidering evidently do not respond to the stimulus of gravity 

 in the same way that an erect stem does. On the contrary, 

 they tend to grow at right angles to the direction of the force 

 of gravity. But when an axillary branch or the end of the 

 stem grows upward, it is because that part of the plant 

 responds in the same way that the trunk of a tree does — 

 namely, by growing in a direction opposite to that of the 

 force of gravity. 



219. Other Forms of Stems and Branches. — The stems 

 of most seed plants come within the classes that have been 



described : erect stems, 

 trailing stems, climbing 

 stems, and horizontal un- 

 derground stems. But 

 now and then a stem or 

 a branch takes on a form 

 different from any of 

 these. Some of the less 

 common forms have al- 

 ready been mentioned. 

 They are always of such 

 nature as to meet a 

 special need of the plant. 

 Most stems contain tis- 

 sues in which food is 

 stored. This is especially 

 true of thick, fleshy stems 



4- v^i^^-^ 



Fig. 121. — The "cotton tree," whose 

 swollen stem serves for the storage of 

 reserve food. After Kerner. 



