carried considerable distances, as in cocklebur, sandbur, tick trefoil, cosmos, 

 and Spanish nettle. 



From Generation to Generation When we think of the lowest plants 

 and animals, we cannot make a sharp distinction between parents and offspring. 

 In the simplest organisms, as we have seen, a whole life span is included be- 

 tween one cell-division and the next. During this lifetime there is very little 

 change in structure: the youngest resemble the oldest in almost everything 

 but size (see illustration, p. 10). The "mother" cell goes out of existence 

 at the moment the "daughter" cells come into being: parents and offspring 

 cannot exist at the same time. 



Among the larger seaweeds the expanding vegetative plants bear special 

 reproductive organs on some of their branches, and discharge tremendous 

 numbers of eggs and sperms into the water. For every pair of gametes that 

 conjugate, thousands are destroyed. For every zygote that starts a new in- 

 di\idual, thousands are destroyed. The mosses and ferns retain the female 

 gamete within the body of the parent until it is fertilized. 



In many species of mosses each green gametophyte ripens but a single egg, 

 and then it nourishes the nearly parasitic sporophyte to maturity. But then 

 one sporophyte discharges thousands of spores (see page 387). The ferns seem 

 about to have soK'ed the problem of li\'ing on dry land. The sporoph)'te has 

 come to be the prominent generation, with expanded green foliage, with 

 stems having definite conducting vessels and mechanical structures, and with 

 fairly good roots. The gametophytes, as we have seen, are flat little plates of 

 cells (see illustration, page 387). These plants depend upon a wet season only 

 for the short period during which the sperms swim out and reach the egg 

 cells. The fertilized egg starts out well nourished within the body of the 

 gametophyte. The expansive sporophyte contributes to the species a vast 

 number of spores, with the chance that the wind will carry a few to spots 

 favorable to starting new gametophytes (see illustration, p. 412). 



Infancy in Seed Plants^ Among the most complex plants, structures and 

 behavior seem to be still further adapted to the advantage of the species. 

 Spores are produced in relatively small numbers. The gametophytes are 

 trivial, one-celled structures that remain dependent upon the parent sporo- 

 phyte. It is through the structures of the sporophyte that pollen spores are 

 enabled to reach a spot suitable for germination. And the parent sporophyte 

 also furnishes the structures through which the pollen tube (male gameto- 

 phyte) reaches the female gametophyte. 



The fertilized egg remains within the wall of the gametophyte, but since 

 this is within the ovule, it is nourished not by the "parent" but by the "grand- 

 parent" — the sporophyte. And the food which the seed accumulates is also 

 supplied by the grandparent. The fertilized egg is nourished until the new 



iSee No. 6, p. 415. 

 411 



