30 



THE CELL 



isolated fragment of protoplasm cannot acquire a cell-wall or survive 

 as an independent cell unless it contains at least one nucleus. The 

 author has also observed, that when a filament of Vaucheria is cut in 

 two, the chloroplasts are withdrawn from the vicinity of wound, whereas 

 the numerous small nuclei, on the contrary, remain in close proximity 

 to the new cell-wall which proceeds to develop over the wounded 

 surface. 



Closely related to the last-mentioned phenomenon are the facts 

 recorded by the author concerning the position of the nucleus in 

 growing cells. In such cases, namely, the nucleus usually lies in more 



or less close proximity to the region of most 

 active, or of most protracted growth ; this statement 

 applies to the growth of the cell as a whole, as well 

 as to the special processes of thickening or surface- 

 extension of the cell-wall. Occasionally a direct 

 communication is maintained between the nucleus 

 and the region of active growth by means of pro- 

 toplasmic strands. In those epidermal cells, for 

 example, in which the outer walls become much 

 thicker than the lateral and inner walls, the nucleus 

 generally lies in contact with the outer wall to begin 

 with, whereas it may shift on to one of the other 

 walls, when the growth of the cell comes to an end. 

 In Aloe verrucosa the external wall of each foliar 

 epidermal cell is provided with a minute solid 

 papilla, which first makes its appearance in the 

 young cell in the form of a sharply defined cushion- 

 like thickening of the membrane (Fig. 3 b). The 

 nucleus places itself in close contact with this 

 cushion, and it remains there until the thickening of the outer wall is 

 completed (Fig. 3 c) ; subsequently it often assumes a different posi- 

 tion. The epidermal cells of pericarps and seed-coats not infrequently 

 have specially thickened inner walls ; here again the nucleus is often 

 (e.g. in Carex and Seopolina, Fig. 3d) found lying in contact with the 

 wall which is undergoing growth in thickness. 



Where growth in surface is restricted to a definite portion of the 

 cell-wall, the nucleus likewise often lies in close proximity to this 

 growing area. In Pimm sativum and in many other plants the pro- 

 trusion of the outer wall of a rhizodermal cell, which represents the 

 first visible stage in the development of a root-hair, always takes place 

 exactly opposite the position of the nucleus (Fig. 3 a). Later the 

 developing root-hairs exhibit pronounced apical growth, and the nucleus 

 accordingly almost invariably takes up its position close to the apex. 



Fio. 2. 



Cell from a hair of 

 Sicyoi angulatus, in 



which the protoplast has 

 become divided into 

 nucleated and non-nu- 

 cleated halves ; of these, 

 only the former has se- 

 creted a new cell-wall. 



