SPERMATOPHYTA 



339 



except that a small ephemeral ventral canal cell is formed. A prominent 

 beak arises between the archegonia, a feature also of the Cordai tales 

 (Fig. 289). 



Male Gametophyte. The development of the male gametophyte 

 begins with the cutting off of two prothallial cells, of which the first is 

 ephemeral, the second persistent. The remaining large cell divides 

 unequally to form the generative and tube cells. This is the shedding 



Fig. 290. Early embryogeny of Ginkgo biloba, X75. A, free-nuclear stage; B, cellular 

 stage; C, later stage with meristematic tissue at lower end. 



condition of the pollen. The further development of the male gameto- 

 phyte takes place as in the Cycadales. The pollen tubes are extensively 

 branched and function as haustoria. The generative nucleus gives rise to 

 the stalk and body nuclei, but these are not separated by a cell wall. A 

 body cell is organized and divides to form two sperm mother cells, within 

 each of which a large swimming sperm is formed. The sperm has a band 

 of cilia like that of the cycads. 



Embryo. In the development of the proembryo of Ginkgo, the first 

 stage is one of free-nuclear division, 256 nuclei being formed (Fig. 290 A). 

 This is followed by a stage of wall formation, the entire egg becoming 

 filled with tissue (Fig. 290B). The lower third of the proembryo now 

 becomes meristematic, while the upper two-thirds remains dormant (Fig. 

 290C). The growing region gives rise to a short massive suspensor and a 

 terminal embryo, the latter developing two cotyledons. On the whole, 

 the embryo of Ginkgo is primitive. The ripe seed of Ginkgo is brownish 

 yellow and about 2.5 cm. in diameter. 



