igo2] ROOT-HAIRS, CAP, AND SHEATH OF AZOLLA 415 



root may be briefly recapitulated. In illustration two of his 



{fig^ 



fih 



large cell near the recurved tip of the growing stem beneath, at 

 one side of the median plane, and in close proximity to an incip- 

 ient lateral bud. This cell is the root initial {fig. i, ri). It abuts 

 internally upon cells from which later the vascular system of the 

 stem originates. It is covered externally by a tabular cell, the 

 root-sheath initial (si), which subsequently divides once peri- 

 clinally(;f^^. 7. 2, s, s') , and eventually in other directions. While 

 at the very first the sheath is thus two-layered, the inner layer 

 very soon disintegrates, and the mature sheath is only one cell 

 thick. As stated above, this sheath in the condition of the plants 

 seen by Strasburger envelops the root completely, though 

 loosely, throughout life. 



From the root initial (ri) a pyramidal apical cell is organized 

 {fig. 2, Ac). A single cap cell is cut off (ci), which afterwards, 

 according to Strasburger, divides but once periclinally, a two- 

 layered, adherent cap thus being instituted. 



^^g- 3 of the accompanying plate represents a very young 

 root. It is enveloped by a sheath comprising a single layer of 

 cells. Shortly after the stage shown, this sheath in all the cases 

 seen by me ceases to grow, and the root, continuing to elon- 

 gate, bursts out. The old sheaths are to be seen as short col- 

 lars, less than a millimeter long, around the bases of the roots. 



The cap, as seen in fig, j, is composed of two very similar 

 cell layers {c\ c»). In roots slightly more advanced a sharp 

 differentiation of these layers is seen to have taken place. The 

 outer becomes radially thicker, and its cells are vacuolated; 

 while the inner remains relatively thin, and its cells are well 

 filled with contents staining heavily like those of the body of 

 the root. These differences forecast the very unlike histories of 

 the two layers at a period a little later still, when the outer one, 

 no longer growing longitudinally, becomes detached from the 

 stem at the root's base, and being loosened from the root except 

 at the tip is borne as a distinct cap ; while at the same time the 

 inner continues to grow and remains in connection with the root 



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