1902] ROOT-HAIRS, CAP, AND SHEATH OF AZOLLA 417 



{Jig, 4, Ti) arise within a belt of actively dividing cells, lying 

 immediately under the inner root cap, not far from the apex, at 

 an actual distance varying with the rate of growth of the termi- 

 nal region. When that growth is most rapid the longest space 

 intervenes between the apical cell and the youngest hair initials. 

 As the root reaches the limit of its development, the hair-form- 

 ing impulse travels downward until the apical cell itself is split 

 into several parts, each one piliferous. The initials are formed 

 in zones or partial zones. The mitoses to which they owe their 

 existence are peculiar in that the axis of the mitotic figure in 

 each case diverges more or less from the longitudinal axis of the 

 mother cell, so that the resulting cell plate and wall lie some- 

 what diagonally {figs, 4, 8), Division thus gives two more or 

 less wedge-shaped elements, of which the lower and slightly 

 larger one is the hair initial. 



This cell never elongates much in a direction parallel to the 

 length of the root. The tube, which presently begins to grow 



out, turns toward the root apex {figs. 5, 6), As the hairs 

 lengthen they at first lie appressed to the root and may be seen 

 confined by the inner cap, which is now distended and pushed 

 away from the root trunk {fig, g). The whole cap structure is 

 finally thrown off through the growth of the lower hairs, and 

 the hairs themselves stand out strongly {figs. 12, ij). Their 

 bases retain the wedge-form of the original hair initial. 



Soon after their organization the hair initials are to be dis- 

 tinguished from the cells with which they alternate by their con- 

 tents, no less than by configuration, since they are more richly 

 supplied with protoplasm {fig. 5), 



The lesser wedge-shaped cell produced simultaneously with 

 the hair initial elongates and soon divides transversely {ftgs, 

 6, g) once, twice, and often three times. The trichomes in 



each longitudinal cell row thus become separated by two, four, 

 or eight cells. The intervening cells may be six, however. 

 The number of divisions in neighboring rows may be different; 

 and through the resulting displacements the original regularity 

 of the hair zones is destroyed. 



In the root of A. caroliniana the differentiation of the tri- 

 chomes proceeds as in ^. filicnloidcs. 



