70 



FOUNDATIONS OF BIOLOGY 



from its typical structure and take over more or less of the 

 functions of other parts. 



For the purpose of propagation, creeping stems occur 

 such as the surface RUNNERS of the Strawberry, and the 

 underground RHIZOMES of many Sedges, Grasses, and 

 common Ferns. Sometimes the stem to a large extent re- 

 places the root system, but more often 

 it acts as an underground reser- 

 voir in which food material is stored 

 up during the short growth period for 

 the rapid development of the flower- 

 ing shoot. This is well seen in some 

 of the early spring Flowering Plants of 

 New England such as Bloodroot and 

 Trillium. (Figs. 31, 39.) 



Again, the stem is greatly short- 

 ened to form a BULB or a CORM; types 

 particularly common in plants adapted 

 to dry soil. (Fig. 32.) Extremely arid 

 regions are characterized by plants, 

 such as the Cacti, in which the leaves 

 are completely suppressed to prevent 

 rapid evaporation ; their function being 

 taken over by the stem which is pro- 

 vided with well-developed chlorophyll- 

 bearing tissue. Sometimes parts (branches) of the stem 

 may superficially resemble a leaf by being flattened or other- 

 wise modified, as in the Prickly Pear, the apparent leaves of 

 the so-called Smilax (Myrsipmfllum) , and the filamentous 

 'leaves' of Asparagus. (Fig. 33.) Finally, the versatility of 

 the stem is illustrated by the thorns of the Honey Locust, 

 the twining tendrils of the Grape, and the tuber of the 

 Potato which is essentially a 'concentrated rhizome.' 



FIG. 32. Bulb of a Hya- 

 cinth, in section, showing 

 roots, stem, bases of leaves 

 of previous year stored with 

 food, and new foliage leaves 

 about the flower cluster. 

 (After Figurier.) 



