46 THE ALGAE 



but a twelve hour period of darkness is necessary to get maximum 

 photosynthesis on the following day. 



Asexual reproduction in H. reticulatum is by means of numerous 

 uninucleate zoospores, which swarm in the parent cell about day- 

 break and then come together to form a new coenobium which is 

 subsequently liberated, further growth being brought about by 

 elongation of the coenocytic cells. It has been found that zoospore 

 production in this species depends not only upon light intensity 

 but also upon its duration : 



24 6 8 10 hrs. of illumination 



500 lux o 6 88 100 1 of cells producing 



4000 lux II 87 100 100 100 J zoospores 



It is interesting to note that the arrangement of the daughter cells 

 in the parent coenocyte conforms to the mechanical laws for ob- 

 taining the greatest rigidity with the maximum economy of space. 

 Zoospore formation in the other species of Hydrodictyon does not 

 appear to occur, or if it does it has not yet been observed. 



Sexual reproduction in all three species is isogamous* and the 

 plants are monoecious. In H. patenaeforme the zygote is motile for 

 a short time, but in the other two species it is always non-motile. 

 At germination the zygote enlarges and divides meiotically to give 

 four* biflagellate swarmers which, after coming to rest, develop into 

 polyhedral cells. These later divide to produce zoospores that are 

 Uberated into a vesicle in which, after a period of motility, they 

 come together to form a new coenobium. Gamete and zoospore 

 reproduction can be induced in H. reticulatum by varying the ex- 

 ternal conditions artificially. If plants are cultured in weak maltose 

 solution in bright light or in the dark and are then transferred to 

 distilled water, zoospores will develop from the light plants and 

 gametes from the dark ones. 



The vegetative plant must be regarded as haploid and its develop- 

 ment is of considerable interest. Further increase in the size of the 

 colony is probably impossible for purely mechanical reasons. 

 Hydrodictyon is essentially a collection of a number of individual 

 plants because it arises as the result of the fusion of a number of 

 swarmers. Volvox, on the other hand, must be regarded as a single 

 plant composed of many cells because it arises from a single zygote 

 or gonidium. 



* 4-8 swarmers or i if from an azygospore. Gametes are antiogamous in 

 H. africanum (Pocock, i960). 



