THE VASCULAR CRYPTOGAMS 



103 



a.n. 



\em. 



It is remarkable that while the Horsetail plant bears 

 no resemblance whatever to a Fern plant, the prothallus 

 and sexual organs are so much alike in both. It is true 

 that they differ a good deal from those of the Male Fern, 

 but other Ferns come very near the Horsetails as regards 

 their sexual generation. 



The conditions of life of the sexual prothallus are, as a 

 rule, simple and uniform compared with those to which the 

 asexual plant is exposed, 

 so that the former has 

 less need for varied struc- 

 tural adaptations. Conse- 

 quently we often find that 

 at this stage there is much 

 in common bet ween families 

 which, so far as their sporo- 

 phytes are concerned, have 

 lost all traces of relation- 

 ship. 



The most striking point 

 about the prothallus of the 

 Horsetails is its being 

 usually (though not without 

 exception) dioecious. We found that in the Ferns very 

 small prothalli often form male organs only, while the 

 better-grown individuals produce archegonia as well. 

 In Horsetails this difference has gone further and 

 become more constant. Even in Horsetails, however, it 

 is not fixed, but depends a great deal upon nutrition. 

 ProthaUi grown on a bad soil (e.g. damp sand) will only 

 produce male organs, while those which are better treated 

 and provided with plenty of food (say in the shape of a 

 food-solution such as that described in Part I. p. 202) 



Fin. 47. Equisctum maximum; 

 fertilised archegonium. a.n, neck 

 ofarchegonium ; em, young embryo, 

 showing first divisions. Magni- 

 fied about 150 diameters. (After 

 Buchtien.) 



