1893. HYBRIDITY IN PLANTS. 211 



■commonest Algas will fructify only in conditions which are unfavour- 

 able to the production of their vegetative organs. From an 

 exceedingly interesting series of observations made by Klebs on 

 that beautiful organism the " water net," Hydyodictyon reticulafum, we 

 learn that all the cells in a net are apparently equivalent, i.e., are 

 adapted to produce either non-sexual zoospores or sexual zooga- 

 metes, and that the tendency to produce one or other of these 

 structures is largely a question of nutrition. In a single net, consist- 

 ing of equivalent sister-cells, some of the cells can be excited, by 

 external conditions, to develop zoospores, others to produce zooga- 

 metes, the formation of the former being, in this case, absolutely 

 dependent on light. A similar law prevails in Volvox. 



Professor Weismann, in his recently-published volume of 

 " Essays," advocates the view that the old idea of an essential 

 difference, at all events of an opposition, between male and female 

 elements, between so-called " sperm-cells " and " germ-cells," must 

 be abandoned ; that they are essentially alike, and differ only so far 

 as one individual differs from another individual of the same species ; 

 and that fertilisation is no process of rejuvenescence, but merely a 

 union of the hereditary tendencies of two individuals. This view he 

 supports from the results of experiments which he regards as demon- 

 strating the remarkable fact that, even in animals as high in the scale 

 as the Batrachia, the " sperm-nucleus " can be made to play the part 

 of an "ovum-nucleus," and vice versa ; it being possible to produce a 

 free-swimming larva in the formation of which no " germ-nucleus " 

 has taken any part ; but, as far as the vegetable kingdom is con- 

 cerned, the weight of evidence does not appear to be in favour of 

 Weismann's hypothesis. 



As has already been pointed out, very early in the evolution of 

 the lower forms of vegetable life the motile zoospores began to 

 assume sexual functions, and to become converted into zoogametes, 

 and these again into active male and passive female elements ; and, 

 although the plasticity which is characteristic of the lower forms of 

 life renders this differentiation for a time unstable, it becomes 

 gradually more and more firmly established. Even in that class of 

 beautiful Algae, the Conjugatse, of which Spirogvva and Zygnema are 

 examples, where the " conjugating " cells show scarcely any difference 

 even under high magnifying powers, it has now been shown that there 

 is an essential difference between the male and female cells in their 

 physiological constitution, and in their behaviour towards one another. 

 Still more decisive is the fact, established by Auerbach in the animal, 

 and by Rosen in the vegetable kingdom, that the male and the female 

 nuclei behave differently to staining reagents, the former belonging 

 to the class of " cyanophilous," the latter to the class of " erythro- 

 philous " substances ; that is, they take up especially blue and red 

 colouring substances respectively. Other histological differences 

 have also been pointed out between the male and female nuclei, 



