242 



STRUCTURE AND VITAL PROCESSES OF PLANTS 



The leafy crown of the tree thus becomes a hollow shape support- 

 ed by the bare branches like an umbrella with its ribs. 



8. Metamorphosed Stems. 



Stems sometimes have special functions other than those 

 mentioned on page 232, and in such cases their structure and 

 shape are generally suited to their special functions. 



The leaf-like swollen stems of Opuntia (p. 57) for example, \^ 

 take over the function of the leaves which are absent during the 

 greater part of the year. At the same time they form a store 

 of water and food. Similar structures are found in several 

 species of the Cactus and Spurge families. 



As stolons and runners they are a means of vegetative propa- 

 gation. These are branches with long internodes creeping along 

 or under the ground and shooting out roots at their nodes, thus 

 terming new stations at a distance from the mother plant. ^The 



Fig. 219. — Stolons of llifdrocolyle. 



illustration (tig. 219) shows a plant, common on the brink of 

 paddy-tields. The strong plant in the middle is the mother 

 })lant, wliich has sent out horizontal runners or stolons forming 

 young plants from their nodes. Other examples of stolons are 

 Spinifex (p. 179) and Potato (p. 91). 



