DICOTYLEDONS 59 



Spermatophytes, and they are subdivided as above, 

 according to the absence or presence of an investing 

 ovary or fruit-wall. 



Angiosperms, with which this book is concerned, 

 differ further from the Gymnosperms in having no 

 endosperm till after fertilisation, whereas Gymno- 

 sperms possess endosperm before fertilisation has 

 taken place. 



Angiosperms possess true flowers, of which the 

 essential parts are the stamens and pistil. The 

 stamens bear pollen-sacs, which are equivalent to 

 the microsporangia of the Pteridophyta amongst 

 Cryptogams, and the carpels bear ovules which are 

 equivalent to the megasporangia. 



The pollen resembles the microspores of Gymno- 

 sperms and Pteridophytes. The ovules are, however, 

 as has been said, enclosed in carpels. There are 

 two integuments (or one), and there is usually one 

 embryo-sac in the nucellus, with one ovum at the 

 micropylar or upper end, which is not, as in some 

 Cryptogams, enclosed in an archegonium. The 

 three antipodal cells occupy the other end, repre- 

 senting another egg-apparatus, in this case function- 

 less. There is a large nucleus in the centre, which 

 gives rise to the endosperm after fertilisation, and 

 this fills the sac. 



The Chalazogamse differ somewhat from other 

 Angiosperms in the mode of fertilisation. 



Angiosperms are also divided into two groups, 

 according to the character of the cotyledons. 



