neo: HYBRIDITY IN PLANTS. 211 
commonest Algz 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,” Hydvodictyon reticulatum, we 
learn that all the cells in a net are apparently equivalent, i.c., 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 Alga, the Conjugate, of which Sfivogvva 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, 
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