Lecture IX. 73 



quiescent cell being regarded as the female gamete or ovum. 

 However, before the process begins, the two are not visibly 

 differentiated. It will often be noted that all or most of the 

 cells of one filament behave as sperms in relation to an ad- 

 joining filament. Thus' we may say that the filaments are 

 male and female. In certain species of Chlamydomonas the 

 gametes are of different sizes and the larger are supposed to 

 be the ova. 



Where there is a marked differentiation between the sexual 

 cells the fusion process is usually spoken of as fertilisation. 



Another point naturally arouses speculation when the conjuga- 

 tion of Spirogyra is observed. What can evoke the outgrowth 

 of the conjugation-tubes? How are they directed towards one 

 another ? It would be highly unsatisfactory to attribute this 

 growth to some mysterious telepathy, action at a distance. As 

 was previously pointed out our knowledge of the ultimate structure 

 of protoplasm and of the transformations of energy which take 

 place within it, is so vague that in these cases of so-called vital 

 action we cannot hope to see the direct connection of cause and 

 effect ; but at any rate we may look for the stimuli which evoke 

 certain responses. The cells, so far as we know, have no 

 visual organs. In this case the stimulus is very possibly a 

 chemical one. The adjacent cells may liberate some substance 

 into the water which is perceived by both or they may, as it has 

 been suggested, deprive the surrounding water of something, e.g. 

 carbon dioxide, and the absence of this acting as a stimulus may 

 evoke as response the growth of the conjugation-tubes. If this 

 last surmise is founded on fact we would have another instance 

 of starvation acting as a stimulus in sexual reproduction. 



Again, it is conceivable that the production of the conjugation- 

 tubes is the direct result of the development of a cellulose 

 softening enzyme .which requires the addition of a certain 

 concentration of a soluble diffusable substance (co-enzyme) to 

 make it efficient. This co-enzyme may be supposed to diffuse 

 out in all directions but only attains sufficient concentration 

 between the filaments. It is therefore only on the adjacent 

 aspects of the cell-walls that sufficient softening occurs to secure 

 the yielding of the wall to the internal osmotic pressure. The 

 first beginnings of the conjugation-tubes are the result of this 

 yielding. The subsequent direction of growth of the prominence 

 (or bulge) thus produced is determined by the region of greatest 

 concentration which will evidently be towards the corresponding 

 outgrowth of the opposite cell. The observed fact that a current 



