36 BOTANY OF CROP PLANTS 



vertically elongated, but they do not reach any consider- 

 able size. 



Functions of Phloem Elements. — The functions of these 

 three elements of the phloem are as follows : 



1. Sieve Tubes. — Conduction of soluble carbohydrates, 

 amido-acids and proteins. 



2. Companion Cells. — Although sieve tubes lose their 

 nuclei before the end of the first year, they do not die; hence, 

 it is thought that companion cells extend their influence to 

 the sieve tubes, enabling them to carry on the Hfe processes 

 for which a nucleus seems necessary. 



3. Phloem Parenchyma. — The cells of this region store food 

 material or conduct it short distances in the stem. 



Cambium. — The cambium layer is composed of one or 

 more rows of small cells, flattened in planes that run at 

 right angles to a radius of the stem. They are thin- walled 

 cells, rich in protoplasm, and capable of rapid cell division 

 and growth. The cambium is in fact the growing layer of 

 the stem. In grafting, one stem, the scion, is inserted 

 into another stem, the stock, in such a way as to bring the 

 two cambium layers together. The cells of these layers 

 possess the power of growth, and after a time there is a 

 union. 



Xylem (Wood). — The chief structural elements of the 

 xylem or wood portion of the vascular bundle are: tracheal 

 tubes, tracheids, wood fibers and wood parenchyma. The 

 tracheal or water tubes are long, large, tubes with thick 

 walls. They have been formed by the elongation and en- 

 largement of rows of cells, the common end walls of which 

 have totally or partially dissolved, leaving a duct of consider- 

 able length. The" walls of the tracheal tubes become thick- 

 ened, and the thickening material {lignin) is laid down on 

 the inside of the walls in various patterns. 



