THE PLANT AS A WORKING MACHINE 13 



and let it remain for a few minutes, and then remove it and 

 examine it by sectioning, definitely stained regions would be 

 seen, other regions being unstained, thus showing not only 

 that the liquid passed upward through the leafstalk, but that 

 it passed through certain tissues of the stalk. If a leafstalk 

 of celery is carefully broken and one part pulled slowly away 

 from the other, there are seen fibers, or threads, which are 

 quite like those shown in the cornstalk (fig. 7). These threads 

 are known as fibrovascular bundles, which means simply " col- 

 lections of thread-like tubes." It is through these fibrovas- 

 cular bundles that water and substances in solution in water 

 pass from the soil through roots, through stems, and into 

 leaves. Through them also plant foods may pass from leaves 

 downward through the plant. Indeed, there are certain parts 

 of each bundle through which water passes upward, and other 

 parts through which the organized plant foods are carried. 

 The fibrovascular bundles, therefore, are the chief transporta- 

 tion lines of the plant. 



11. Leaves. Most leaves are expanded so that they expose 

 much more surface than would stems of equal weight. In 

 some cases (fig. 3) the entire leaf is expanded, while in others 

 there is a leafstalk, or petiole, and the expanded portion, the 

 blade. The leaf blade may be single (simple) or sub-divided 

 (compound). From the plant stem the fibrovascular bundles 

 extend into the leaf, where they are known as the -veins of the 

 leaf. They terminate in the leaf, sometimes in its tip and 

 sometimes in the margin as well as in the tip. Water from 

 the soil may therefore pass through the fibrovascular bundles 

 of the roots, the stem, and the leaves, into the interior of the 

 leaf. From the leaf some of this water is evaporated into 

 the air. 



Part of the water in the leaf, instead of being evaporated 

 into the air, is used in the construction of plant food by means 

 of a process which is of very great importance to all living 

 things. Carbon dioxide from the air enters the leaf through 

 its surface. The leaf is green because of the presence of a 



