562 



HUTCHINSON'S POPULAR BOTANY 



FIG. 709. BLADDER WRACK. 



Antheridia. Antherozoids are seen escaping from : 

 antheridium. 



referred to as forming one of the 

 elements in the composition of 

 Lichens belong to this order. More 

 strangely still, some of them (Zoo- 

 chlorella) are found living in the 

 cells of low forms of animal life, 

 such as the Hydra, the Fresh- water 

 Sponge, certain Planarian worms, 

 and Infusoria, for whom they manu- 

 facture starch, which animals are 

 incapable of doing. Another 

 method of reproduction found in 

 this order is by the union of two 

 naked motile masses of protoplasm, 

 known as planogametes. which be- 

 come fused together and form a zygote, which develops into a cell-family 

 enclosed in a common envelope. The well-known Volvox, so long bandied 

 about from zoologist to botanist and mce versa, is by some considered to be 

 a hollow colony of single-celled Protococcids, each with its pair of cilia pro- 

 jecting through the common envelope, whose movements give that revolving 

 motion to the colony that has delighted all who have viewed it through 

 the microscope. Within the colony smaller daughter-colonies may be seen 

 revolving; also aggregations of ciliated spermatozoids. Large colonies 

 may contain as many as twenty-two thousand individuals, each connected 

 to five or six neighbouring cells by delicate threads of protoplasm. Certain 

 of these cells develop into large egg-cells, which are fertilized by the 

 spermatozoids. These egg-cells may be distinguished from the daughter- 

 colonies by the lack of motion. When the egg-cells are mature, the old 



colony breaks up into its con- 

 stituent cells, and the egg-cell (now 

 an oospore) sinks to the bottom, 

 and in a resting condition develops 

 into a new colony. In the Order 

 ConfervoideaB the individuals are 

 multicellular, but in most cases the 

 cells are in a single row. placed 

 end to end, and form long fine 

 threads. Some are fresh-water 

 species, some marine, whilst one 

 Trentepohlia is aerial. They are 

 reproduced asexually by the pro- 

 toplasm of certain cells breaking 

 up into swarm-spores. There are 

 two forms of sexual reproduction. 



FIG. 710. BLADDER WRACK. 



Section of a conceptacle, showing several ooeonia amon 

 antheridial hairs. 



