292 TAYLOR— ON THE PRODUCTION OF 



the underlying cells may bud out into the pith cavity. That the growth 

 is not limited to the elongation of one or of a few adjacent cells is shown 

 by Plate LXXVII, Fig. 24. 



Coincident with the condition just described, there was another and 

 more important development. This was not generally preceded by any 

 proliferation of the pith cells into the cavity, it being strongest when the 

 inner cells, bordering on the cavity, had been killed by the injection. 

 It consisted of the formation of a cambioid zone from pith cells. This 

 has been already described for Ricinus, but not for the other species 

 worked upon by the writer. The procedure was as follows : — 



The cells underlying the dead area first elongate radially and tend 

 in so doing to decrease the size of the internal cavity. Then they divide, 

 and the resulting division walls are fairly transverse to the direction of 

 elongation (Plate LXXV, Figs. 13, 14, 15). 



Next, the cells in a portion of this prohferating region divide more 

 rapidly than the others, thus forming a meristematic zone which takes 

 the form of a sheath often wholly surrounding the part of the cavity 

 touched by the injected fluid. The nuclei in this meristem and in the 

 cells of the proliferated region are often unusually prominent. The 

 protoplasm is not dense, as in an embryonic layer in a growing point, 

 but is scant, as in the cambia of ordinary stems. 



Sometimes the cells just inside of this meristem have the appearance 

 of having been cut off from it. As they are often yellowish, and stain 

 readily with Safranin, it might be thought that they were cork, but 

 repeated applications of tests for suberized membranes have failed to 

 demonstrate its presence. 



An interesting fact in this connection was found in Polygonum. In 

 some of the untreated plants there had appeared deep slits or furrows 

 in the pith. About the bases of these sHts, near the protoxylem, the 

 pith cells showed a tendency to divide, and the areas thus formed re- 

 sembled the early stages in the formation of the pith-cambium. 



The question now arises as to what is the true nature of the meri- 

 stematic zone. Is it a true cambium, forming both phloem and xylem, 

 or does it only form xylem? There is first formed a belt of cells that 

 are like xylem parenchyma, there being gradually added to these in the 

 later formed zone, pitted vessels. The formation of this zone seems 

 to start near the insertion of the needle, and progress around the stem 

 from there. The form of the elements is not as regular as in the normal 

 wood, nor are they as large, but they are undoubtedly xylem elements. 

 However, cells with thickened pitted walls were seen in some of the 



