376 



PART IV. VEGETABLE TAXONOMY. 



sperms, also, the pollen-grain, before it is quite ripe, develops, in 

 some cases, two cells in its interior, one with a curved, vermi- 

 form nucleus, the other with an elliptical one ; but the two cells 

 do not usually become enclosed in cellulose walls ; and when the 

 pollen-tube is formed, the only indication of their former exist- 

 ence that remains, is the presence of the two nuclei still to be 

 seen within it. Only one of these, however, the worm-shaped 

 one, appears to take part in fertilization ; the other is the 

 vegetative one, and undoubtedly represents the prothallium. 

 See Fig. 574. A B 



Fig. 574. — Pollen of Tradescantia Virginica. A, showing small vegetative and larger 

 vermiform generative nucleus. In B, it will be seen that the generative nucleus belongs to 

 a distinct cell, which is smaller in size than the vegetative cell. C, shows the pollen-grain 

 germinating, and the generative nucleus passing down into the tube. The Figures after 

 Coulter and Rose. 



The female prothallium, formed in the embryo-sac of Angio- 

 sperms, is much more rudimentary than in Gymnosperms, being 

 represented only by the three cells developed in the base of the 

 embryo-sac, called the antipodal cells. (See Fig. 296, p. 100). It 

 may be that the three cells, constituting the egg-apparatus, cor- 

 respond to the archegonium of Gymnosperms, but this point is 

 yet unsettled. It is only after fertilization is effected that an 

 endosperm is developed in this group of plants. It serves the 

 same purpose as the endosperm formed in the embryo-sac of 

 Gymnosperms previous to fertilization, and is called by the same 

 name. It is obviously, however, not the same organ, but is rather 

 to be regarded as a new formation. 



The Class includes two markedly distinct sub-classes, the 

 Monocotyledons and the Dicotyledons. 



