VISIBLE CONSTRUCTIVE ACTIVITY IN PROTOPLASM. 



581 



together again into two fresh nuclei. After they have performed this function, 

 these spindle filaments have a further and no less important part to play. Almost 

 at the identical place where the nuclear plate was previously to be seen, an 

 accumulation of exceedingly small granules arises, the repeatedly mentioned micro- 

 somata; and these are arranged to form a plate, the so-called cell-plate, which 

 ultimately divides the whole cell into two compartments. Apparently these spindle- 

 threads serve also as conductors to these microsomata, and maiij' of tlie small 

 granules are conveyed along them to the equator. But occasionally they arise 

 there directly, and help to produce the cell-plate. The development of the cell- 

 plate does not seem to be always quite the same in different species, but it is 

 establi-shed with certainty that in it cellulose micellaj are always formed, and that 

 the partition- wall produced from them possesses the characteristics of a cellulose 

 wall, that is to say, of a cell-wall. Already it has been mentioned (p. 44) that in 



x._ 



-^ 



1 



Fig. 135.— Changes in the Protoplasm of the Cell-nucleus during its Division. 



» The nuclear fibrils distributed through the whole nucleus. 2 The broken-up nuclear fibrils arranged as the nuclear plate. 

 * The elements of the plate separating from one another. * The same elements forming two skeins at the poles of the 

 spindle. (After Guignard.) 



this cell-wall, at least at first, albuminous portions of protoplasm are retained by 

 means of which the intercalated membrane can undejrgo manifold metamorphoses, 

 and that by them, if required, the communication is maintained between neighbour- 

 ing masses of protoplasm. 



In the cells of those green water-threads known as Spirogyra, Zygnema, and 

 Cladophora, as well as in those of desmids and many other simple plants, the 

 plants never seem to come to an end of this dividing. Each cell continues to grow 

 until it has attained certain dimensions; it then divides into daughter-cells in the 

 manner peculiar to it, and in these the process which has been performed in the 

 parent cell is repeated afresh. This process continues perpetually under favourable 

 external conditions, and an interruption occurs only when there is lack of necessary 

 food, or when the living protoplasm is killed by unfavourable circumstances. In 

 these plants, of which we can enumerate more than a thousand different species, 

 there is thus no distinction into portions in course of formation and those which 

 have been completed, and are no longer capable of development. It is otherwise 

 in large plants in which a division of labour and a corresponding organization 

 have taken place, in those plants whose different members perform diflferent 

 functions. In these, stability of some members is of the greatest advantage, and 



