STEMS 29 



underground and aerial stems are possessed by such common 

 plants as Irish potato, onion and asparagus: 



Water is also a medium of growth of stems, as is the case 

 with such plants as Elodea, Potamogeton, water hUes, etc. 



"Modified" Stems. — Undoubtedly, the ordinary cyUn- 

 drical twig such as is found in trees and shrubs is the most 

 common sort of stem. It is quite hkely that we think of a 

 stem as a plant part growing more or less erect, in fact, most 

 stems do tend to grow erect. However, all stems are not as 

 just described. As we take a survey of the plant kingdom, 

 we discover many different forms of stems — stems that are 

 so different from the ordinary sorts that they are scarcely 

 recognizable as stems, and are identified as such, only by 

 careful study of their origin and structure. Among such 

 stems are the following : 



I. Rootstocks or Rhizomes (Fig. 12). — These are under- 

 ground, horizontally elongated stems. Thfe rootstocks or 

 rhizomes of Canada thistle are excellent examples. They 

 bear reduced, scale leaves at the nodes. Lateral buds arise 

 in the axils of these leaves, just as described in the cotton- 

 wood twig^ — a typical stem. They grow in length from a 

 terminal bud, which is unprotected by tough scales. Adven- 

 titious roots are produced at the nodes. Rootstocks are 

 efficient organs in the spreading of a plant. Here is a 

 method of reproduction other than by seeds. Usually, 

 aerial stems from the lateral buds of the rootstock are pro- 

 duced; they may die back to the ground each fall. The 

 plant lives over the winter by means of the rootstocks. 

 Hence, rootstocks or rhizome-bearing plants are perennial. 

 Many of our worst weeds are perennials from a rootstock. 

 We may prevent such plants from going to seed, but in spite 

 of this, and the cutting back of the leafy shoots, new shoots 

 are sent up from the rootstocks. Furthermore, if the root- 



