THE GENEEAL BTEUCTUBB OF PLANTS 9 



FIG. 10. COLONIES OF Protococcus. 

 X 750. 



of these units of construction, a variety which finds its 

 expression in the multiplicity of existing forms, and the 

 differences of dimensions which various organisms exhibit. 



The simplest plants, as we have seen, are unicellular, 

 and many remain in this condition throughout the whole of 

 their existence. When they have attained a certain size 

 the cell or protoplast divides into two. Sometimes these 

 two become separated from each other, and we have two 

 plants where but one existed before. Plants with this habit 

 remain unicellular, and the 

 division of the cell is equiva- 

 lent to the reproduction of the 

 plant. The unicellular condi- 

 tion in other cases is transitory, 

 and the plant soon comes to 

 consist of two, four, or more 

 cells, in consequence of the 

 products of each division re- 

 maining attached together. 



We get in this way a small colony of cells, each like the others 

 both in structure and in function. When the power of 

 division is limited the resulting colony consists of a limited 

 number of cells, and is often found surrounded by a common 

 cell- wall or membrane. This condition is seen in such 

 plants as Chroococcus, Protococcus, and other humble Algae 

 (fig. 10). A colony of somewhat higher type, though still 

 of microscopic size, is found in the form of a hollow sphere 

 (fig. 11), the wall of which is one cell thick (fig. 11, A). This 

 organism, known as Volvox, shows a little higher differentia- 

 tion than those last described, the cells being furnished 

 with cilia by means of which the little sphere can propel 

 itself through the water. 



In other cases the association of a number of protoplasts 

 is not complicated by the formation of any cell-wall. Fig. 

 4, A shows an aggregation of a number of naked proto- 

 plasts which have combined to form a plasmodium. These 

 organisms are found creeping about upon moist surfaces ; 



