34 



DIFFERENTIATION OF THE TISSUES 



ing toward maturity the procambium-like cells enlarge, and princi- 

 pally in the vertical direction, so that the ends shove past each other 

 and become pointed; the walls thicken and become lignified as a 

 rule (Fig. 15), and the protoplasts finally disappear, leaving the 

 fibers dead. This is the mode of origin of bast fibers wherever 

 they occur. 



The bast fibers seem to serve chiefly, if not solely, for giving 

 strength, for which purpose they are fitted, whether they occur in 



the primary cortex or in the pericycle, 

 by their vertical elongation, thick and 

 lignified walls, and dove-tailing or in- 

 terlacement of the ends. In roots the 

 primary cortex usually consists of thin- 

 walled parenchyma alone (Fig. i6j. 



The innermost layer of the primary 

 cortex is the starch sheath or endodermis 

 (Fig. 14). This in stems is, as its 

 name implies, unusually rich in its 

 starch content, and other than this, in 

 stems, it oftentimes has no striking 

 characteristics. In recent years good 

 evidence has been brought forward, 

 notably by Haberlandt, to show that 

 the starch grains in this tissue act like the otoliths in the 

 ear, and by falling always to the lower side of the cell as the 

 position of the part to which it belongs is shifted, they furnish 

 by their impact the stimulus for perceiving the direction of the 

 gravity pull. 



In other instances, and particularly in roots, the endodermal 

 cells become differentiated from the rest of the primary cortex 

 by elongating somewhat in the vertical direction, suberizing their 

 radial walls, and by partially or completely thickening their 

 walls (Fig. 16). 



The intercellular spaces that can be found in other tissues of 

 the primary cortex are lacking in the endodermis. These charac- 

 teristics have led to the conception that the endodermis possessing 



Fig. 16. — Portion of a cross, 

 section of a root of Allium 

 ascalonicum. h. large central 

 tracheal tube; i, xylem, and /, 

 phloem portion of the vascular 

 bundle; n, cortex cells; j. endo- 

 dermis with thin-walled cell at 

 k to admit passage of materials; 

 m, pericycle or pericambium. 

 (After Haberlandt.) 



