THE NUTRITIVE ORGANS OF PLANTS 



45 



protoplasm permits the inward flow 

 of the soil-water, but practically 

 prevents the outward flow of the 

 cell-sap. Thus we see that proto- 

 plasm has a selective action. Since 

 the growing parts of roots have 

 countless root-hairs, these cells of 

 the epidermis together act like a 

 great sponge which absorbs the large 

 quantities of water and mineral mat- 

 ters which are needed by all plants. 

 This liquid passes from one cell to 

 another until it reaches the central 

 cylinder. A study of the micro- 

 scopical structure of the central cyl- 

 inder makes evident the fact that 

 this part of the root consists not 

 only of tough wood cells as explained 

 in the preceding section, but also of tubular cells called ducts. 

 (See Fig. 14.) Through these ducts the sap is conveyed up- 

 ward to the stem. 



FIG. 14. Ducts that convey 

 the sap upward through 

 the root, stems, and leaf. 

 The walls of these ducts 

 are strengthened by spiral 

 fibers or rings. 



II. THE STRUCTURE AND ADAPTATIONS OF STEMS 



59. The structure of a woody stem. Laboratory Study 

 No. 31. 



A. The structure of a young stem. 



Secure pieces of a young stem of a horse-chestnut, maple, 

 lilac, or other woody stem that shows the three 

 layers of bark. Split some pieces lengthwise in 

 halves. 



1. Peel off the outer covering, the bark, from a piece of 

 the stem till the wood is exposed. The bark of 



