THE STEM 239 



culm of a Rice plant we have seen that the sides of the stem are 

 exposed to the greatest stress and the middle portion to the 

 least; hence* the culm is hollow to save material and the sides 

 are strengthened by a cylinder of strong fibres. Similarly the 

 stem of the Labiata3 and many other plants have their four corners 

 strengthened by strong cells. The strength of beams is propor- 

 tional to the breadth and to the square of the depth. This rule 

 is illustrated by the edgewise placement of leaf-ribs, e. g., in the 

 Teak, or of plank- roots (sec fig. 207). If other plants are exam- 

 ined in this respect, we always find that, though the position of 

 their fibres may be modified, they always answer the fundamental 

 principles of architectural structure. 



When the crown of a tree is shaken and the stem is bent by 

 a storm, the roots have to sustain an enormous pull, just like 

 the cables that are used to keep a vessel at anchor. If such 

 cables were untied and their several strands were made to hold 

 the ship, they would be easily torn one by one by the movements 

 of the vessel. As they are united into cables, the pull which 

 would tear the single strands is equally distributed over every 

 one of them, and thus the cable is able to Avithstand it. Thus 

 in roots we find the woody and bast-cells crowded together in 

 the middle, which makes tlieni strong like cables. Similar 

 arrangements are found in the climbing stems which are also 

 exposed to strong pulls. 



6. Various Types of Stems. 



The greater the load of leaves a stem has to bear, the stronger 

 must be its structure. Herbs are comparatively small plants of 

 a year's growth, or of perennial growth, if their underground 

 parts remain alive at the time when their overground shoots 

 perish. Such herbs have soft or herbaceous stems. Hollow stems, 

 articulated by solid and swollen nodes, as we find them, in 

 Grasses, are termed culms. If a stem consists of one long inter- 

 node only, and bears no leaves but flowers only at its top, as in Cri- 

 num, it is called a scape. The stem that does not die at the end of 

 a season, but lasts for years, becomes woody. If such a woody 

 stem branches off from the ground, it is called a shrub (Ixora 



