REPRODUCTIVE CELLS. 



139 



on absorbed, whilst the final developing embryo grows at the 

 expense of the prothallium contained in the macrospore : more 

 than one oosphere is usually fertilised, but the embryos formed 

 from these do not get beyond a certain stage. 



JVole.— The practical examination of the reproductive cycle in the 

 Gymnosperms is readily carried out by taking longitudinal sections of 

 male and female cones of Pinus at various stages of their growth. The 

 development of the microspore can be made out by this method, and the 

 maturation is often to be observed when the pollen-grain is lying at 

 the apex of the nucellus of an ovule in the female cone. 



The formation of the embryo-sac and its prothallium may be studied by 

 taking sections of very early female cones of Pinus from the time when 

 they are about 4 mm. in length onwards. Archegonia are soon formed 

 after the prothallium is complete, but some care is necessary in selecting 

 cones for this examination. 



Fertilisation and subsequent changes are best seen in sections of ovules 

 of cones gathered from June 1st to the end of that month, although, in 

 some Conifers, fertilisation takes a long time. The later changes, 

 just previous to and after the elongation of the suspensors, are more 

 readily obtained than the earlier ones. In Pimis fertilisation takes two 

 years to accomplish. 



C. Reproduction in the Pteridophyta. 



In this great group of plants, as was pointed out above, an 

 alternation of g^euerations is met with — that is, a sexual 

 generation formed from the spore alternates with an asexual 

 generation which arises as the result of the fusion of the two 

 effective cells produced in special organs upon the sexual plant 

 or gametophyte. Moreover, two main tjpes of reproduction are 

 met with in Pteridophyta. In the one, or Homosporous type, 

 only one kind of spore is produced, and this, by its germination, 

 produces the gametophyte or prothallus ; whilst in the other, or 

 Heterosporous type, two kinds of spore are found — viz., micro- 

 spore and macrospore — in each of which a separate prothallium 

 is formed. Thus, the microspore in the latter case produces a 

 male prothallium upon which organs comparable to antheridia 

 arise in which the male effective cells arc formed, and the 

 macrospore produces a female prothallium upon which an organ 

 arises in which the female effective cell originates. 



The Homosporous type is exemplified in Pteris or AqniUam. 

 Avhich belong to the order Filicinese ; and the Heterosporous 

 type is seen in Marsika or Sak'mia, members of the order 

 Hydropteridese. Each of these types will be examined in order. 



