CELL-DIVISION AND DEVELOPMENT. 



127 



the cases among plants which have led Sachs to this impor- 

 tant deduction. Sachs recalls the cases of such algae as Can- 

 lerpa, in which an organism of considerable size, differentiated 

 into root-, stem-, and leaf-like portions, is presented, the whole, 

 nevertheless, consisting of but a single cell, just as a unicellu- 

 lar Infusorian may reach a size considerably greater than that 

 of many multicellular Rotifera, and may show also a very 

 decided differentiation of tissue. The most conclusive case 

 bearing upon the idea is, however, that of the alga Stypocaulon, 

 a portion of which is represented in Fig. i. 

 Here one finds in the terminal part of 

 the laro-est branch a distinct indication of 



o 



branching, and it may be noticed that the 

 terminal portion of the branch is equal 

 in size to the lower portion. In other 

 words, this terminal part of the branch 

 has reached its full growth and yet it has 

 undergone no cell-division. Growth is in 

 this case accordingly independent of cell- 

 division, and the converse is likewise true, 

 for in the older portion of the plant, in 

 zvJiicJi groivtJi has ceased, cell-division 

 occurs. 



The form of the plant, accordingly, since 

 form is dependent on the mode of growth, 

 is not determined by the cell-division; but, 

 on the other hand, it seems that, to a cer- 

 tain extent at least, the mode of cell- 

 division is determined by the form, an idea which has been 

 admirably expressed by DeBary in the aphorism, " Die Pflanze 

 bildet Zellen, nicht die Zelle bildet Pflanzen." A simple ex- 

 ample of this thesis is furnished, according to Sachs, by the 

 pollen mother-cells of the orchid Neottia. These cells vary 

 in shape considerably, and the mode of their division into the 

 four pollen grains to which each gives rise, varies according 

 to the shape. Thus where the mother-cell is circular and 

 discoidal it divides into four cells lying in a single plane 

 (Fig. 2, A) ; where it possesses an oval form two cleavage 



Fig. 



(After Sachs.) 



