90 



PRACTICAL COURSE IN BOTANY 



Fig. 102. — Thorn branches of 

 Holocanthn Emoryi, a plant growing 

 in arid regions. 



is so much used for decoration. 

 The deUcate green blades are 

 merely altered stems, shortened 

 and flattened to simulate leaves. 

 102. Weapons of defense. — 

 Conspicuous examples of these 

 are the bristling thorns of the 

 honey locust. Is their frequent 

 branching any indication of their 

 real nature ? Does it prove any- 

 thing, or must you look for other 

 evidence? WTiat further indi- 

 cations might you expect to 

 find, if they are true branching 

 stems? (100.) On old haw, 

 plum, crab, and pear trees, stems can be found in all stages 

 of transition, from stubby, ill-developed branches, to well- 

 defined thorns. 



103. Storage of nourishment. — This is 

 one of the most frequent causes of modifi- 

 cation in both roots and stems. Of stems 

 that grow above ground, the sugar cane 

 probably comes first in economic importance 

 on this account. In hot, arid regions, where 

 the moisture drawn from the earth would, 

 during prolonged drought, be too rapidly 

 dissipated by an expanded surface of leaves, 

 the whole plant, as in the case of the cactus, 

 is sometimes compacted into a greatly thick- 

 ened stem, which fills the triple office of leaf, 

 stalk, and water reservoir. 



104. The uses of underground stems. — 

 It is in these that the storage of nourishment 

 most frequently takes place, and the modi- 

 fications that stems undergo for this purpose 

 are iji some cases so great that their real 



Fig. 103. — Melon 

 cactus, showing 

 greatly condensed 

 stem for the storage 

 and preservation of 

 moisture. 



