196 THE ORCHID REVIEW. [JULY, 1915, 
eet ay 
Ea THE MECHANISM OF HEREDITY. te 
(Continued from page 186.) 
E saw last month that hybridisation consists in the combination of 
specifically distinct ancestries in the same individual, the necessary 
result being a compromise between the conflicting qualities and the 
production of individuals possessing more or less intermediate characters. 
We also saw that hybridisation itself is only an extension of sexual 
reproduction, the latter a process which differs from vegetative reproduction 
in certain definite particulars, the essentials of which are the production of 
two kinds of specialised cells, which are incapable of further growth until 
after their fusion in pairs in the sexual process. But sexual reproduction 
is not universal, being entirely absent from certain primitive groups of 
plants, and the fact invites inquiry as to the very origin of sex. The fact 
is, sexual production is itself an adaptation, it is something which is added 
on to the earlier process of reproduction by division, and the fact should 
help us to understand some of the highly specialised processes of 
reproduction among the higher plants. The very names Schizophyta 
and Conjugate, which have been applied to certain primitive groups, are 
based upon this fundamental distinction in their method of reproduction.. 
It is even held that sexual reproduction has arisen independently more than 
once, but this point is immaterial for our inquiry. 
It will facilitate an understanding of the two processes if we take a 
simple case, as in the filamentous Alga known as Ulothrix zonata. Here 
vegetative reproduction takes place by the production of what are known as 
zoospores ; pear-shaped bodies, containing a single nucleus, and furnished 
with four long cilia, by which locomotion in water is brought about. 
These arise by bipartition from the ordinary cells of the filament, and swim 
about for some hours before they come to rest, and attach themselves to 
some object by the small end, after which they grow out into a new 
Ulothrix filament like the parent. But other cells, it may be of the same 
or of different filaments, produce bodies of a somewhat different kind. 
They also are produced by bipartition, but differ from the other kind of 
zoospores mentioned in having only two instead of four cilia, and in their 
smaller size. There is no other visible difference between them. Their 
behaviour, however, is different. They are, in fact, the sexual cells of 
Ulothrix. On escaping from the mother-cell they swim off through the 
water just like the larger kind of spore, but on meeting with another two- 
ciliated zoospore, produced by a different mother-cell, an extraordinary 
process takes place. The two at first become entangled by their ciliz, and 
then goon spinning through the water together; their bodies now come 
