KEPRODUCTIVE CELLS. 



153 



Fertilisation takes place by the passage of an antherozooid 

 into the oosphere, a plug of mucilage containing a cliemical 

 substance secreted b}- the latter acting as a means of attraction 

 for the antherozooids (positive chemotaxis) ; fusion then occurs, 

 and the result of this is an oospore, which soon assumes a thick 

 wall of cellulose. After a period of quiescence the wall of the 

 oogonium ruptures and frees the spore, which germinates, form- 

 ing a typical Vaudieria filament. 



Note. — The study of the reproductive processes in the AlgiL- is often best 

 carrier! out by first growing filaments, &e., in an aquarium, so that plants 

 may be gathered and examined at frequent short intervals. Sjnrogyra 

 "conjugates" towards autumn as the water is getting colder, and so is 

 unfavourable for vegetative reproduction The same applies to VKiirJif/nn. 



In Fucus, the conceptacles are found in the club-shaped swollen ends of 

 certain fertile branches of the thallus, and externally look like small "pits" 

 or dimples in the surface of these. .Sections may betaken in the transver.se 

 direction, and these will often cut the conceptacles at the sides of the branch 

 in a direction perpendicular to the surface. 



Antheridia and oogonia in Fiirni arise in separate conce|)tacles. 



The Homology of the Various Types of Reproduction. 



By the term Homology, used in connection w ith reproduction, 

 is meant a comparison of the various stages in the reproductive 

 cycles of different groups of plants, and is interesting from the 

 fact that there may often be traced in the higher types studied, 

 remnants of phases whicli are more or less marked and uf 

 importance in the cycles occurring in lower types. The study of 

 Homology is thus the only reliable method of placing a plant in 

 its correct position in the scale of evolution, and as such, should 

 be given due consideration in the study of Botany. 



The comparison of tlie various reproductive cycles which have 

 been examined is best made by drawing up a table, showing, in 

 each group, the successive stages met with during maturation "f 

 the primary sexual elements, up to the time when fertilisation is 

 completed by the union of the effective cells produced during 

 the maturation process in each element. Such a table would be 

 somewhat as is seen in the table of Homologies, facing p. 154, 

 the male and female elements being distinguished by the 

 symbols ^ and 5 respectively. 



From this table it may be seen that in Angiosperms, the 

 antipodal cells, formed during the maturation of the embryo-sac 



