178 PLANT-LIFE 



through which, two generative cells (male elements) pass 

 without losing their way, and one of them fertilizes the 

 ovum. It is the superior machinery of the Phanerogams 

 that enables them to economize, and we need not 

 wonder that a division of plants so ingeniously equipped 

 is now dominant in the vegetable realm. 



The critical reader, while admitting the importance 

 of the facts just adduced in their bearing upon the evolu- 

 tion of Flowering plants, may argue that it is a far cry 

 from the sexual generation of a Fern to that of a Wall- 

 flower, or from a free, ciliated spermatozoid to the en- 

 closed generative cell; he will ask for " missing links" 

 which indicate transitional phases between the great 

 extremes. Without venturing upon an elaborate dis- 

 cussion, one such missing link of special interest may be 

 mentioned — a link which decidedly connects the Pteri- 

 dophytes and the Seed-producing plants. This link is 

 found among the Cycads, which resemble Palms and 

 Tree-Ferns in general appearance, but are marked off 

 from them in important particulars: the point of 

 moment to us just now is that in all the Cycads so far 

 investigated the pollen grain on germination forms a 

 pollen tube from which two swimming spermatozoids are- 

 discharged ; these motile male elements actually swim in 

 a watery fluid contained in a cavity of the ovule, in their 

 effort to reach the archegonium and effect fertilization. 



The Cycads are the lowest of the seed-bearing plants ; 

 they are not numerous now, less than eighty species 

 being in existence, but in the geological Middle Ages 

 they were very abundant, as the fossil remains of extinct 

 species show. The existing species are confined to the 

 warmer regions of the world; some of them form a 



