174 



INTRODUCTION TO CYTOLOGY 



The Cell Wall. — Probably the most striking difference which meets 

 the eye in comparing animal and plant tissues is the degree of distinctness 

 with which the limits of the individual cells can be made out. Animal 

 cells, as a rule, are separated only by very thin membranes, which in 

 many tissues are so delicate as to be scarcely discernible; whereas the 

 cells of plants usually possess conspicuous firm walls, which, in the case 

 of woody plants, become greatly thickened and afford mechanical support 

 to large bodies. 



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Fig. 101. Fig. 102. Fig. 103. 



Fig. 101. — Continued extension of partition wall after completion of mitosis in endosperm 

 of Physostegia virginiana. (After Sharp, 1911.) 



Fig. 102. — Delimitation of generative cell in pollen grain of Scirpus. (The dark 

 bodies are degenerating nuclei of grains which fail to develop.) (After Piech, 1928.) 



Fig. 103. — Embryonic cells in Ephedra, developed by "free cell-formation" in a com- 

 mon mass of protoplasm. (After Land, 1907.) 



As shown in the foregoing section, the development of the cell wall in 

 vascular plants ordinarily begins with the formation of a fluid cell-plate 

 at the close of mitosis. Cytologists are not yet in agreement regarding 

 the exact relation of this layer to the primary wall layer, or middle lamella 

 (the "intercellular substance" and "cement" of early waiters). For 

 some time it was thought^' that the cell-plate became the middle lamella 

 directly, secondary and tertiary layers being deposited upon it by the 

 protoplasts on either side. Soon it was suggested, ^^ as already stated, 

 that the cell-plate splits, the middle lamella then being deposited between 

 its halves. As we have indicated (p. 171), the young cell-plate has been 



12 E.g., by Strasburger (1875, 18826, 18846). 



" Treub (1878). Strasburger adopted this view in 1898. It was supported by 

 the work of Timberlake (1900), C. E. Allen (1901), and a number of later writers. 



