24 



BOTANY 



B 



tooth Violet (Erythronium) are familiar examples of common wild 

 flowers with bulbous stems, and in the dry regions of our Pacific 

 coast, as is true in other similar regions, the number of bulbous plants 

 is very great. The beautiful Mariposa Lilies (Calochortus), Brodisea, 

 Fritillaria, among others, may be mentioned. In our gardens, too, 

 many plants with corms and bulbs, like the Crocus, Gladiolus, Tulip, 

 Hyacinth, Narcissus, etc., are familiar examples. 



Another modification of the stem, in plants of dry regions, is illus- 

 trated by the Cacti and other so-called Xerophytes. In these, protec- 

 tion against drought is effected by a reduction of leaf-surface, which 



T^w-"-* _. i n extreme cases 

 results in a complete 

 suppression of the 

 leaves. In such 

 plants the stem 

 develops a large 

 amount of green tis- 

 sue which is protect- 

 ed by a very thick 

 epidermis, or masses 

 of hairs. Parts of 

 the stem may be- 

 come flattened and 

 resemble a leaf also 

 in form. Thus the 

 flattened joints of 

 a Prickly Pear or 

 the apparent leaves of the gardener's " Smilax," and the threadlike 

 "leaves" of Asparagus, are really all modified stems (Fig. 15). 



Stems may be modified, for the purpose of climbing, in two ways. 

 Either the whole stem may twine as it does in a Morning-glory or 

 Hop, or branches may be changed into tendrils, like those of the 

 Grape or Virginia Creeper. 



Creeping stems, like the runners of the Strawberry, or the under- 

 ground " Stolons " of Mint and many Grasses, are stems modified for 

 purposes of propagation. 



Thorns developed for protection against attacks of animals are 

 often modifications of stems. The great branched thorns of the 

 Honey-locust show their cauline nature very clearly, often, when 

 young, having leaves growing from them like those from normal 

 shoots. 



The Leaf 



The normal leaves of vascular plants, while exhibiting a great 

 diversity of form, agree in the main in their essential structure. The 



FIG. 15. A, leaflike shoot of a Cactus (Cereus). B, leaf- 

 like shoots (phylloclades) of Myrsiphyllum. 



