512 LIFE-HISTORIES 
advantage probably results from the union of different protoplasts. 
Much research in recent times warrants the belief that the offspring 
of two parents is benefited by the interaction of the slightly dif- 
ferent powers inherited from either side. Invigoration of the off- 
spring and increased adaptability are often plainly shown. Within 
specific limits, the beneficial effect of a cross, as the union of gametes 
from different individuals is called, has been found to be greater as 
the parents are less alike or have lived under more dissimilar condi- 
tions. Hence plants which can co-operate in the production of off- 
spring while living somewhat apart, make the most successful 
parents. Traveling gametes, as in Ulothrix, enable them to do this. 
But in order to travel well a gamete must be comparatively small, 
and when this is true of both gametes as in Ulothrix, the resulting - 
zygote cannot be large or very well provided with food, and is there- 
fore at a disadvantage in becoming a resting zygospore. Here then 
is an opportunity for a useful division of labor in co-operative repro- 
duction. Let one of the gametes remain small for traveling, and 
let the other become as large and as well stored with food as possible, 
then the result of their union will be a cross-fertilized zygote of 
superior capabilities. The fact that the most highly developed 
groups of alge, notably the higher Chlorophycez and the Rhodo- 
phycez, have adopted this expedient indicates that the experiment 
has been a great success among thallophytes wherever it could be 
fairly tried. Along with the possibility of cross-fertilization is apt 
to go also the possibility of union between gametes from the same 
individual. This is distinguished as close-fertilization. It is better 
than no fertilization at all, but seems scarcely more beneficial to off- 
spring than non-sexual reproduction. 
Where both gametes are set free, about the best that can be done 
is the formation of such zygotes as we find in Fucus, where the off- 
spring receives no further care after fertilization has taken place. 
When, on the other hand, as in Coleochete, only the male gamete is 
set free, the female plant gains the opportunity to act as a nurse to its 
offspring, giving it additional protection and sometimes food after 
fertilization until it is well able to take care of itself. This nursing 
may so affect the development of the offspring that it becomes 
strikingly different from the form which bears it; then we have an 
alternation of generations. It is in this new development, which 
represents the highest achievement of thallophytes in their care of 
offspring, that we shall find potentialities of the utmost importance 
for the further development of plants. 
Fungi, especially non-aquatic forms, have generally degenerated 
so far as to lose any power of fertilization they may once have had. 
This may be because in the more or less isolated situations they 
usually occupy, co-operative reproduction seldom is possible; and 
another important reason may be that their dependence upon 
ready made food throughout life makes invigoration and nursing 
