THE PATH OF DEGENERATION IN PLANTS 



201 



the prothalli are unisexual, but the spores from 

 which they are produced are alike. Finally, in the 

 allies of Selaginella which are the Cryptogams most 

 nearly approaching flowering plants, the prothalli 

 which bear male organs 

 are quite distinct from 

 the prothalli which bear 

 female organs ; more- 

 over, the spores which 

 give rise to the two 

 kinds of prothalli are 

 quite different. A 

 small number of large 

 spores arise in special 

 sporangia, termed 

 macrosporangia, and it 

 is from these that the 

 female prothalli arise. 

 A larger number of 



Smaller Spores are pro- FlQ- 70 ._Secdling of Fern (Asplenium 



duced in 



anm'a anrl HTPCP Tninrn 

 angia ana Lliese micro- 



crnrpa nrnrlnpp tTiP TTIQ!P young seedling developed from the 

 Spores produce llie Iliaie fertilized ovum has already given rise 



prothalli. 



The spores of ferns are very small and contain 

 only the nutritive matter required for germination ; 

 the spores of Selaginella, even the microspores, are 

 very much larger and contain a large supply of 

 reserve material at the expense of which the 

 development of the prothallus takes place. 



miCrOSpOr- The prot hallus Q?) beai s antheridia at its 

 edges and towards the centre an arche- 

 gonium which has been fertilized. The 



