110 POND LIFE. 



is spirally arranged. In each of the cells is a nucleus, and certain 

 fibres or strings, which are the protoplasm — the living matter of the 

 cell — so that it consists of four parts. First, the nucleus (a)^ in the 

 centre of it ; secondly, the living protoplasm (b), round the 

 nucleus ; third, the green colouring matter, which is the same as 

 the green matter of leaves {c); and fourthly, the husk (d), or " cell- 

 wall," which is formed or excreted by the protoplasm. It grows 

 in the following manner : — First of all the nucleus divides, just as 

 in the case of the Amceba. The protoplasm which forms the 

 substance inside the cell also divides, and while this is going on 

 the wall gradually begins to draw in, until at last it forms a division 

 quite across, so that where there was only one cell before, there 

 are now two, which soon attain the same size as the original. 



But more than this may be observed in this plant. Most 

 commonly in the autumn, and at night, where these thread-like 

 plants form a thick mat, two cells, side by side, will put out 

 certain processes, which presently come into contact, and form a 

 tube connecting the tw^o cells (Fig. 19). All the substance from 

 one cell collects into a ball, and passes over into the other 

 (Fig. 20). After this it forms an outer coating, like the AvicEba^ 

 and remains thus throughout the winter (Fig. 23.) In the spring it 

 buds out again, to form a new filament like the parent (Figs. 21, 22), 

 so that this plant has the power of doing two different actions. 

 First, by the process of " cell-division," it grows, that is to say, 

 producing a larger plant ; and, in the second place, by means of 

 two elements which unite together, it forms an entirely new plant. 

 By the one process the plant is enlarged, and by the other a 

 new one is produced. 



There is another interesting plant which may be found pretty 

 commonly in ponds about Edinburgh, and is called Volvox 

 globator (PI. XII., Fig. 11). It is always turning and twisting itself 

 about when alive, by means of a number of hair-like threads, 

 projecting from it on all sides. It is really a colony; that is, 

 each of the little green bodies which bear the hairs is a cell, 

 and the whole round body a mass of cells. Inside it you may 

 notice a small yellow globe, covered with wart-like projections 

 resembling a raspberry. Each wart afterwards becomes separated, 

 and then has two fine hairs. In the colony there are also other 



