SEXUALITY OF PLANTS 33 



to the substratum, as are Ulothrix or Fucus, or even 

 rooted in the soil like Ferns and Flowering Plants, effectu- 

 ally prevents their movements as a whole from place to 

 place. In respect of sexuality this imposes a vital 

 difference. The mobile Animal is free to seek its mate ; 

 the encysted and rooted Plant is not. Hence the whole 

 problem of sexuality for the Higher Plants appears to 

 be a different one from that of the Higher Animals. 

 Nevertheless in both a very similar coalescence of gametes 

 is the end to be attained, and there are various analogies 

 in the means employed to attain that end. 



In Plants that live in the water the fact that all except 

 the simplest are non-motile as a whole, and fixed to the 

 substratum as Seaweeds are, does not present any serious 

 obstacle to success. For they are mostly gregarious, and 

 one of the gametes or both are commonly motile in the 

 water into which they escape. In those which are 

 sexually differentiated the male commonly retains, as a 

 spermatozoid, its power of movement from place to place : 

 and Algae so provided are believed to represent the remote 

 ancestry of the Land- Vegetation. There is no need in 

 such cases for both gametes to be motile, if the gamete 

 which is sedentary can control the movements of that 

 which is motile. That it can do so is demonstrated 

 by any mixture of water containing living spermatozoids 

 of Fucus with water containing its ova (Fig. 10 (5) ). The 

 influence of attraction before syngamy and of repulsion 

 of the remaining spermatozoids after syngamy suggests 

 that the power of the ovum lies in diffusion from it into 

 water of some soluble substance, attractive or the reverse. 

 A study of the cognate phenomenon in Frns has shown 

 that this is a true explanation. But it would only serve 

 for organisms in which a water-medium is available at 



S.H. C 



