THE GENEEAL STEUCTUBE OF PLANTS 9 



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 

 condition in other cases is 

 transitory, and the plant soon 

 comes to consist of two, four, 

 or more cells, in consequence 



. -, j p -, -,. . FIG. 10. COLONIES OP Proto- 



of the products of each divi- coccus, x 750. 



sion remaining attached to- 

 gether. 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, Proto- 

 coccus, 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 differentiation 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 protoplasts 

 which have combined to form a plasmodium. These 

 organisms are found creeping about upon moist surfaces ; 

 they form the group known as the Myxomycetes or slime- 

 fungi. One species, Mthalium, is found frequently among 



