THE CASTOR BEAN 



107 



ends and others have ends tapering to a point, the cells dove- 

 tailing to form the hard, resisting part of the stem; the phloem 

 outside the cambium layer also contains several kinds of cells. 

 Some of them are large and have oblique ends which are per- 

 forated by apertures that place one cell in communication 

 with the next above and below. Because of these openings, 

 these cells are called sieve cells. It is through these cells that 

 the food supply is transported through the plant from the 

 leaves. Close to the sieve cells are smaller cells, the accom- 

 panying cells, a, which are long and slender. The phloem 

 also contains many rather narrow cells with square ends called 

 parenchyma cells, and a few small, short cells with very thick 

 walls known as stereome cells. 



The same longitudinal sec- 

 tion shows that the pith, p, 

 is made of short, square cells 

 with very thin walls. Evi- 

 dently the pith is a soft tis- 

 sue and the strength of the 

 stem is due to the hard and 

 resisting fibrous cells in the 

 bundles. Outside of the 

 bundles, directly beneath the 

 epidermis, it will be seen that 

 the cells of the cortex, co, are 

 much like those of the pith, 

 hardly longer than they are 

 broad, with thin walls and 

 square ends. 



The relation of the fun- 

 damental cells to the fibro- 

 vascular bundles is better 

 shown in Figure 47, which shows how the bundles extend 

 through the stem and strengthen it. The bundles evidently 

 consist of very different material from that found in the pith 



FIG. 47. PERSPECTIVE VIEW OF A 

 PIECE OF A YOUNG STEM, SHOW- 

 ING THE FIBROVASCULAR BUNDLE 

 EXTENDING LENGTHWISE IN THE 

 STEM FOR SUPPORT 



co, cortex; 

 ep, epidermis; 



fb, fibro vascular bundle; 

 p, pith. 



