50 



PLANT BIOLOGY 



also be supplied with water and other materials from the 

 soil. Our experiment with red ink (see 61) showed that the 

 soil-water is carried upward through the woody portions of 

 stems. A microscopical examination of thin sections of 

 a stem (see Fig. 15) shows the presence of tubular cells 

 known as ducts, similar to those found in 

 the central cylinder of roots with which 

 they are connected. These are the 

 parts of the wood through which the 

 soil-water passes most readily up to the 

 leaves. 



After the raw materials have been 

 changed into the plant foods by green 

 leaves, these plant foods, by the process 

 of digestion, are changed into such a 

 form that they can pass from the leaves 

 into the fibrous bark in which are tubular 

 cells known as sieve-tubes. (See Figs. 15 

 and 16.) Through these the liquid food 

 passes down the stem to be stored away 

 or used in the growth of root or stem. 

 In young stems the pith rays or medul- 

 lary rays (59, B, 3), the fine lines extend- 

 ing from the bark toward the center of 

 the stem, are supposed to serve as chan- 

 nels for the passage of food across the 

 stem and also for the storage of food. 



B 



FIG. 16. Sieve tube, 

 that conveys sap 

 downward through 

 the leaf, stem, and 

 root. A, longitu- 

 dinal section show- 

 ing edge view of 

 sieve plate (in the 

 middle) ; B, sur- 

 face view of sieve 

 plate. 



In the type of stem represented in the corn, lilies, and palm 

 trees, the woody material through which sap passes is not ar- 

 ranged in the form of annual rings, but the woody bundles are 

 scattered through the pith. Each bundle consists of ducts that 

 carry the soil-water up through the stem out into the leaves, of 

 sieve-tubes that convey downward from the leaves the manufac- 



