128 Science of Plant Life 



while the dicot fibers are made up of only the strand of bast 

 cells from the food-conducting side. The bundle sheaths 

 are usually thicker in the bundles near the outer part of the 

 monocot stem. In fact, in some monocots like rattan and 

 bamboo, the sheaths of adjacent outer bundles may join each 

 other and thus form a hard layer beneath the epidermis 



(Fig. 73)- 



The structures of conifer stems. The conifers, like the di- 

 cots, have their bundles arranged in a hollow cylinder. In 

 structure these bundles are somewhat similar to those of 

 dicots, except that the wood and water-conducting cells are 

 not distinct. The wood cells form the water-conducting tis- 

 sue as well as the mechanical tissue. In keeping with their 

 double function, the cells (tracheids) are thick-walled and 

 spindle-shaped, with numerous thin places, or pits, in two of 

 the walls. Because of this structure, the stem retains its 

 rigidity and still permits the ready passage of water and min- 

 eral salts. 



Growth of stems. The limit of growth of a stem is not so 

 definite as that of leaves. The length and the diameter of 

 a stem depend largely upon the conditions under which the 

 plant lives, the available water supply, amount of light, 

 temperature, and quality of the soil. Along a dry roadside 

 a ragweed may complete its development with a stem less 

 than 6 inches long, while in a rich bottom-land field the same 

 plant might have reached a height of 15 feet. 



The growth in length takes place at the apex of a stem, the 

 growing point being located in the terminal bud. The grow- 

 ing region extends back from the tip, sometimes for only a 

 fraction of an inch, more rarely, as in rapidly growing vines, 

 a foot or two. If we mark the upper portion of a growing 



