126 Plant Life and Evolution 



is still effected by large motile sperms very much 

 like those of the ferns. The only plants in which 

 this has been found are the fern-like cycads, in- 

 cluding the so-called " sago palm " of the green- 

 house, and the curious Ginkgo, or maiden-hair tree, 

 which is not uncommon as an ornamental tree. 

 Both of these types had long been recognized as 

 being very ancient ones and as having very close 

 resemblances to the ferns, and the discovery that 

 they both develop these motile sperms practically 

 makes this relationship certain. In both the cycads 

 and Ginkgo, the female gametophyte is not essen- 

 tially different in its structure from that of the pine 

 or Selaginella, and the pollen grain, after it has 

 fallen upon the ovule, also develops a pollen-tube 

 as it does in the pine. This pollen-tube, however, 

 becomes greatly distended by an accumulation of 

 water and finally bursts, discharging the two enor- 

 mous sperms, together with the water, into the 

 chamber which lies above the archegonia. So we 

 see, even among the seed-plants, there may still 

 be this same aquatic type of fertilization that ob- 

 tains in the whole of the archegoniate series from 

 which these plants have sprung. In a Cuban cycad, 

 which has recently been described by Caldwell, there 

 may be as many as sixteen sperms developed in 

 one pollen-tube. 



In the pine and other similar types, the pollen- 

 spore, in which the germination is well advanced at 

 the time the spore is shed, shows a division into 



