Ch. XIII) THE SEED PLANTS 515 



the sporophyte, acquired originally by the first land plants 

 in adaptation to the need for lifting the spores into the air 

 for their dissemination by the wind, offered an opportunity 

 for the development of lateral food-forming members, and 

 hence for an indefinite increase in height, which was not 

 possible to the creeping habit of the original gametophytes. 

 It is possible, however, that the presence of the double num- 

 ber of chromosomes in the cells of the sporophyte may have 

 had influence in promoting its greater development. 



In many respects the Spermatophytes exhibit no great 

 advance over the Pteridophytes. In differentiation of the 

 plant body and of the tissue systems, the improvement is 

 only minor, presumably because those parts had already 

 attained in the Pteridophytes the fullest functional efficiency 

 of which they are capable. In reproduction, also, the ad- 

 vance is slight except in one particular, which, however, has 

 proven profoundly important, viz. the transfer of the sperm 

 cell from its place of formation to the egg cell is no longer 

 accomplished by free locomotion in water, but by a com- 

 bination of wind or animal dissemination with growth of a 

 pollen tube. This method has rendered the fertilization of 

 the Spermatophytes independent of the presence of stand- 

 ing water which the Pteridophytes require for the locomotion 

 of their free-swimming sperm cells. It is for this reason that 

 the Fern plants must keep a prothallus on ground that is oc- 

 casionally wet, a fact which greatly limits the range of the 

 Pteridophytes. Freed from this need by the pollen tube, the 

 Spermatophytes have been able to extend over much drier 

 country than the Pteridophytes. Moreover, they have been 

 able to keep the prothallus on the sporophyte, and even 

 within the spore itself, where, greatly reduced, it still has 

 part in the formation of egg cell and sperm cell, and acts 

 as a store of nourishment for the embryo plant. 



Ecologically the Spermatophytes are typical terrestrial 

 plants, mostly of the kind called mesophytes (page 190) ; 

 but they have also diverged in their evolution into nearly 



