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 

 s])ecific limits, the beneficial effect of a cro.s.s, 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 imder 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 algse, notably the higher Chlorophyccte and the Rhodo- 

 phycese, have adopted this expedient indicates that the experiment 

 has been a great success among thallophytes where^"er 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 Coleocha^te, 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 thallo])hytes 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 thej' 

 usually occupy, co-operative reproduction seldom is possible; and 

 another imjiortant reason may be that their dependence upon 

 ready made food throughout life makes invigoration and nursing 



