226 The Microscope. 



Usually most cells contain one or more vacuoles filled with 

 ■watery cell-sap. The Dutch botanist De Vries, and his pupil 

 Went, claim to have proved that these also can be traced back 

 to the youngest cells, and like the plastids arise by division of 

 preexisting structures like themselves. This however, is not ad- 

 mitted by all botanists, and requires further investigation before 

 it can be accepted without question. 



In most plants the cells are provided with a distinct cellulose 

 membrane and are clearly separated from one another. Each is 

 provided with a single nucleus and when the cell divides this is 

 accompanied by a corresponding division of the nucleus. 



Sometimes however, we find that the division of the plant- 

 body into distinct cells is incomplete, and that the nucleus may 

 divide without any division of the protoplasmic body of the 

 cell; and this process may be repeated until a large multi-nucleate 

 cell results. If, therefore, we regard a cell, in its strict morpho- 

 logical sense, to be a mass of protoplasm with a single nucleus, 

 these large " cells," are not properly to be regarded as such. 

 Common instances of these multi-nucleate cells we see in the 

 various Algse known as Siphoneae, of which various species of 

 Vaucheria are common examples. Similar structures are the 

 hyphse of most fungi. 



In the lowest plants it is still an open question whether a nu- 

 cleus, in the proper sense of the word, can be said to exist, al- 

 though there is little doubt that nuclear substance is present in 

 all of them. Of these low plants the bacteria and the Algae known 

 as Cyanophycese are familiar to all students of the lower plants. 

 To the latter group belong many familiar forms, among them 

 the different species of Nostoc, one of which is shown in figure 1. 



Here we have an instance of cell-division in its simplest form. 

 A single filament of the plant will usually show all stages in the 

 process (see Fig. 1 ; the difierent stages are indicated by the small 

 figures). It consists simply in an elongation and constriction 

 of the cell, and finally the formation of a delicate membrane 

 that completely divides the daugter-cells which soon become 

 rounded and entirely similar to the original mother-cell. 



In Cladophora, a common coarse branching alga found abund- 

 antly in running water, the process of cell-division is very easily 

 studied. (Fig. 2-7.) The large cells are multi-nucleate and cell- 

 division and nuclear division go on quite independently of each 



