904 
PHENOMENA OF SEXUAL REPRODUCTION, 
which beai's spores. If we compare the histological and the morphological development 
of the two generations, it becomes strikingly evident, among the Vascular Cryptogams 
at least, that the generation which has been developed as the result of fertilisation 
is much more highly organised than the generation (prothallium) which has been 
developed from the spore. In the Mosses it might appear that the contrary is the 
case, for in these plants the sexual generation is the one which grows independently and 
which is differentiated into leaf and stem ; however, the histological differentiation of the 
sporogonium is far more perfect than that of the moss-plant, so that it is true for Mosses 
also that the product of fertilisation is the more highly organised of the two generations. 
All these cases of evident alternation of generations lead to the conclusion that 
a process of development of a more complex kind is initiated by the sexual act. 
To a certain extent this is true also of the Zygomycetes, the zygospore being a more 
highly organised cell than any of those of the mycelium, and it can scarcely be doubted 
with reference to the Coleochaeteae, the Characeae, and the Florideae that the sexually- 
formed spore-fruit is histologically the most complex product of these plants. Although 
the same cannot be said with reference to the Oosporese and to the Conjugatae, still this 
by no means affects the significance of the sexual act for other plants. A complete 
discussion of the facts would, on the contrary, probably show that the higher develop- 
ment of the Pandorineae, of the Conjugatae, and of the Diatomaceae when compared with 
the Protophyta has been probably promoted by the evolution of sexuality, even though 
this is not expressed by a well-marked alternation of generations. The Phanerogams 
afford a similar, though exactly opposite case : in them the alternation of generations is 
. exhibited in only a rudimentary form, for, in the course of the development of these 
plants from some primitive type allied to the Vascular Cryptogams the sexual generation 
(prothallium) has been reduced to its simplest expression. Whereas in the Oosporeae 
the sexual generation is the predominating one and the product of fertilisation is but 
imperfectly developed, in Phanerogams it is the generation produced in consequence 
of a sexual act which comes to be completely developed, and it is the sexual generation 
(Prothallium, Endosperm) which is rudimentary. In the latter case we have the end, in 
the former the beginning of phytogenetic series ; in the latter the alternation of genera- 
tions is disappearing, in the former it is in the first stage of its evolution. If therefore 
we desire to understand the significance of sexuality in the history of the development 
of a single plant or in that of the whole Vegetable Kingdom, we must fix our attention 
upon those groups in M'hich an alternation of generation is evident: in such cases 
(Vascular Cryptogams, Muscineae) the effect of sexuality is obvious. We may then con- 
clude that the coalescence of the male with the female cell causes the development of an 
organism which is more highly differentiated both histologically and morphologically. 
Sect. 33. Influence of the origin of the reproductive cells on the 
product of fertilisation. The male and female cells or the organs that produce 
them are formed at a greater or lesser distance from one another on the same 
plant, or on different individuals of the same species. The male and female cells 
of the same species may thus be more or less nearly related to one another as 
having been immediately or more remotely derived from the same parent-cell. The 
question arises what influence this genetic relationship of the male and female cells 
exercises on the product of fertilisation. At present we are unable to lay down any 
general law in this respect; but the overwhelming weight of evidence points to the 
law that the sexual union of nearly related cells is detrimental to the preservation 
of the plant, and in general the more so the further the morphological and sexual 
differentiation of the species has advanced. Only in a few plants of low organ- 
isation does a fertile union take place between sister-cells, as in Rhynchonema 
among Conjugatae. But in most Algae and Fungi (as Spirogyra, CEdogonium, 
