422 MORPHOLOGY OF GYIMXOSPERMS 



In any event, starting with the known condition among hetero- 

 sporous pteridophytes, the tendency among gymnosperms has been 

 to reduce and finally to eliminate the vegetative (prothallial) tissue, 

 and to reduce the sperm mother cells to two. 



In certain groups (as Abietineae) the prothallial cells are two in 

 number; in others (as cycads) there is one prothallial cell; and in 

 still others (as Taxodineae, Cupressineae, and Taxineae) prothallial 

 tissue has been eliminated. Such prothallial cells as do appear are 

 sometimes persistent and sometimes ephemeral; so that the evidence 

 of a disappearing tissue is complete, and it actually has disappeared 

 in what are recognized as the most modern groups. The situation 

 common to Podocarpineae and Araucarineae is usually cited as an 

 illustration of a more extensive and therefore a more ancient prothal- 

 lial tissue, which connects directly with the "multicellular" pollen 

 grains of Cordaitales. This may be true, but all of the extra cells 

 are derived from two primary ones, which hold a definite place in the 

 ontogeny of the gametophyte; and therefore the extra cells may 

 represent a secondary tissue that holds no phylogenetic relation to 

 the more extensive prothallial tissue of older forms. In any event 

 it is ephemeral, breaking down and liberating its nuclei. 



The number of sperm mother cells is so rigidly two, that this reduc- 

 tion may be said to have been accomplished by all living gymnosperms, 

 whatever may be the fact in reference to the paleozoic gymnosperms. 

 It is interesting to note that the very few instances of a greater number 

 of sperm mother cells occur in a group characterized by its retention 

 of ancient features (Cycadales), and in another group characterized 

 by its very modern features (Cupressineae). While phylogenetic 

 continuity of multiple sperms is safely claimed for Mkrorycas, no 

 such claim could be maintained for Cupressineae. 



The greatest epoch in the history of the male gametophyte of gym- 

 nosperms, however, was the abandonment of ciliated sperms, and this 

 occurred in connection with the establishment of Coniferales. It is 

 not generally appreciated that five of the seven recognized primary 

 groups of gymnosperms possess ciliated sperms, and that the modern 

 type or truly terrestrial type of sperm was introduced by the conifers. 

 It must not be supposed that the earliest conifers necessarily did not 

 possess ciliated sperms, for these may have persisted among the 



