ON HYBRIDIZATION AMONG PLANTS. 
305 
want of affinity between the pollen and stigma to which it may 
be applied, and that the pollen may have sufficient influence 
to cause the ovary to swell, but no ovules to be produced ; in 
other cases the ovules are developed, but no embryo ! So 
that it must be borne in mind, fertilization is not merely one 
distinct act, but that many conditions are combined, each of 
which must be satisfied in order to produce a perfectly 
developed and embryonated seed. To the physiologist hy- 
bridism is valuable as giving him an insight into the changes 
that take place in the anatomy of plant organs, e.g., the 
differentiation and deterioration of pollen, distribution of 
colour, and other interesting points connected with the 
vitality of plants. 
There would seem to be at least three features which cannot 
help striking the observer as apparently affording some clue 
to discover the relative degrees of fertility and sterility of 
hybrids. 
The first is the diversity of form ; secondly, constitutional 
differences ; and thirdly, what might be called different 
idiosyncrasy between the two individuals we may wish 
to blend. The first is that character of plants with which 
the systematist is for the most part alone concerned ; 
for by morphological differences does he build up his various 
groups, and the greater the difference in form, the more 
remote is the affinity assumed to be; and, as a general 
rule, the slighter is the tendency to blend. So that for the 
purpose of experimenting upon the fertility or sterility of 
hybrids, we are compelled, as a rule, to confine ourselves to 
the so-called species, i.e., plants whose supposed affinity is 
very great. 
Another barrier to success in intercrossing allied forms 
is perhaps found in their constitutional differences. By 
this we mean their habits, which are regulated by climatical 
circumstances : so that one plant affects a dry soil ; another 
species allied to it, a humid, or even becomes aquatic. One 
is a herb and another a shrub. One can only live in a high 
temperature, and consequently is impatient of cold ; another 
is hardy. 
Here we would remark that this affords a valuable subject 
for experiments to those interested in the acclimatization of 
plants. For, though it would seem that no plants are ever 
strictly acclimatized, i.e., made to endure a climate which they 
do not naturally experience, yet a tender tropical plant 
might, by being crossed with a hardy species (provided they 
are capable of uniting), produce a half-hardy hybrid, which 
would probably stand a colder climate, and yet retain many 
