98 



PRACTICAL COURSE IN BOTANY 



and back from the leaves to the parts where it is needed after 

 it has contributed to the elaboration of food. 



On account of this double line of communication which 

 they have to maintain, the vascular threads, or bundles, as 

 they are technically called, are double ; one part composed 

 of larger vessels, carrying water up, the other consisting of 

 smaller ones, bringing back the food. Can you give a reason 

 for their difference in size ? 



112. Woody monocotyls. — Examine sections of yucca, 

 smilax, or of palmetto from the handle of a fan, and compare 

 them with your sketches of the cornstalk. 

 In which are the vascular fibers most abun- 

 dant? Which is the toughest and strongest? 

 Why? Trace the course of the leaf fibers 

 from the point of insertion to the interior. 

 How does it differ from that of the fibers 

 in a cornstalk ? 



1 13 . Growth of monocotyl stems. — After 

 tracing the course of the leaf veins at the 

 nodes of the cornstalk, you will have no 

 difficulty in identifying these veins as part of 

 the vascular system. In jointed stems like 

 those of the corn and sugar cane and other 

 grasses, their intercalation between the vas- 

 cular bundles of the stem takes place, as we 

 have seen, at the nodes, forming the hard 

 rings known as joints; but in other mono- 

 cotyls the fibers entering the stem from the 

 leaves usually tend first downward, toward the interior 

 (Fig. 114), then bend outward, toward the surface, where they 

 become entwined with others and form the tough, inseparable 

 cortex that gives to palmetto and bamboo stems their great 

 strength. Generally, monocotyl stems do not increase in di- 

 ameter after a certain point, and as they can contain only a 

 limited number of vascular fibers, they are incapable of sup- 

 porting an extended system of leaves and branches. Hence 



Fig. 114. — Lon- 

 gitudinal section 

 through the stem 

 of a palm, showing 

 the curved course of 

 the fibrovascular 

 bundles (Gray, after 

 Falkenberg). 



