632 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



same mother cell. The actual process of pairing has been fully described, and it 

 may be taken as a type of the course of events, as it has been observed in all Algse 

 with isoplanogametes whose pairing has been fully investigated. Out of the crowd 

 of gametes swarming in all directions, two approach and stroke each other with 

 their flagella; in some cases the two separate and both become again lost in the 

 crowd, but, when pairing is going to take place, they become firmly fixed together 

 by their colourless anterior ends. The long axes of their bodies may then lie in one 

 straight line, or may diverge at a wide angle. A rotation of each of the pairing 

 gametes about its fixed anterior end now always occurs, the bodies becoming gradu- 

 ally approximated, so that their long axes come to lie nearly parallel. Fusion of the 

 protoplasm follows, beginning at the already joined anterior ends, and progressing 

 rapidly backwards till a single mass of protoplasm is formed. The four flagella still 

 move actively, and the Zygozoospore, as this active type of zygote is often called, 

 escapes from the general membrane of the mother colony, becomes spherical by 

 shortening of its long axis, loses its flagella, and puts on a cell- wall. The protoplasm 

 soon loses its green colour, becoming reddish, and the zygote enters on a resting 

 period. Eventually its protoplasm divides, producing zoospores, each of which gives 

 rise to a new colony. 



The division of the cells of a colony to form zoospores or gametes begins in the 

 evening, and is finished soon after sunrise. In dull weather, however, its comple- 

 tion is delayed far into the day. This dependence of the formation of zoospores 

 and gametes upon the influence of light, if not invariable, is found very widely 

 among the Green Algae. 



Pandorina is a fairly common form in ponds, &c. The colony consists of 

 sixteen wedge-shaped cells arranged in a sphere, and covered by a general invest- 

 ment, which is of considerable density at its external surface. The apex of each 

 wedge is directed towards the centre of the sphere, and there is little space left 

 between the adjacent cells. The formation of daughter-colonies is similar to that 

 obtaining in Gonium and Stephanosphcera. The young colonies escape by liquefac- 

 tion of the investing membrane. Colonies of gametes are formed in the same 

 way, but often consist of eight instead of sixteen cells, and the acquirement of 

 flagella and liquefaction of the mother membrane takes place more slowly. Event- 

 ually the membrane of each gamete-colony also becomes liquefied, and most of the 

 gametes swarm out into the water. A great number of mother colonies of different 

 sizes simultaneously take part in this production of gametes, so that the water 

 becomes filled with masses of swarming gametes of very variable dimensions. No 

 distinct size-categories are, however, to be distinguished. Conjugation now takes 

 place between pairs of gametes either of the same or of different sizes; with this 

 exception, that the largest gametes do not fuse with one another. They are rela- 

 tively inactive, sometimes, indeed, remaining fixed in their colonies, and are sought 

 out and paired with by the smaller and more active individuals. Here, then, we 

 have a most interesting stage in the evolution of sex. The largest, relatively 

 passive, gametes may fairly be called female, while the different sizes of smaller 



