46 ELEMENTARY ORGANS OP PLANTS. 



cell. So also in the DiatomaceEe, a species of marine algae, 

 where the union of plant-cells is only temporary, the organs 

 of nutrition and reproduction arc still identical. The cells of 

 these plants are at first united, but afterwards spontaneously 

 disarticulate and break up, exhibiting well-marked spontane- 

 ous movements, insomuch that some naturalists have referred 

 them to the animal kingdom, to which they certainly approxi- 

 mate. Fig. 19 shows this process of disarticulation in Dia- 



toma marinum after the consummation of the reproductive 

 process. The cells thus separated, under suitable conditions, 

 individually develop into new and independent plants. 



54. But when plant-cells unite together permanently, as 

 they do in the higher forms, the organs of nutrition and repro- 

 duction are no longer identical or confined to the same cell; 

 on the contrary, some of the cells are specialized or set apart 

 for nutrition, and others for reproduction. 



This is beautifully exemplified in the Mucor, or bread- 

 mould (Fig. 20), which consists, as to the creeping part at its 

 base, of long, threadlike and branching cells, the partitions 

 of which have been wholly absorbed, so that they form con- 

 tinuous tubes, whilst its upright portion, or stem, is composed 

 of a single row of cells, formed by the process of division 



