130 STUDY OF COMMON PLANTS. 



the spore has left the mother plant, and it is still for some time 

 enclosed in the macrospore, which also contains a large amount of 

 food materials. The whole structure shows a likeness on the one 

 hand to the spores of other vascular cryptogams, and on the other 

 to the embryo-sac of flowering plants. 



Developmental History and Minute Anatomy. 



As in the case of the fern, a laboratory study of the 

 developmental history requires a special investigation ex- 

 tending through some weeks or months. The following 

 important features of the cycle of development may be 

 mentioned: Selaginella, as well as the ferns and horse- 

 tails, is characterized by alternation of the. oophyte, or 

 sexual generation, with the sporophyte, or non-sexual gen- 

 eration. The latter differs widely from that of the ferns, in 

 that instead of one kind of spore, giving rise to prothallia 

 which bear both antheridia and archegonia, there are two 

 kinds, macrospores, or female (archegonia-bearing) spores, 

 and microspores, or male (antheridia-bearing) spores, are 

 produced, a distinct foreshadowing of what is seen 

 in flowering plants, the microspores corresponding to 

 pollen-grains, and the macrospores to the embryo-sac of 

 the ovule. The oophyte, again, as compared with that 

 of the ferns, is reduced in size, and all its early stages of 

 development are completed within the spore, reminding us 

 of similar facts in the developmental history of phanero- 

 gams. The prothallium of the microspore, in particular, 

 is reduced to the low.est terms, and should be compared 

 \vith the two or more vegetative cells (rudimentary pro- 

 thallium) in the pollen-grain of certain gymnosperms. 

 The archegonia, produced only on the prothallium of the 

 macrospore, are essentially like those of ferns, though 

 somewhat simpler, but after fertilization the first septum 

 of the oospore is formed at right angles to the axis 



