478 BOTANY FAIIT 11 



reduction division in the spore -mother -cells of archegoniates and 

 Phanerogams have resulted in a confirmation of the limits of the two 

 generations in the latter ( 2a ). The number of chromosomes charac- 

 teristic of any plant is diminished to one half during the divisions 

 that lead to the origin of the sexual generation, and the full number of 

 chromosomes is not again attained until fertilisation takes place. The 

 asexual generation has always the double number, the sexual genera- 

 tion the single number of chromosomes. The constant progression 

 of the former generation at the cost of the latter might be placed in 

 relation with this difference between them ; a strict demonstration of 

 such a connection is, however, not possible. 



The Spermaphyta are divided into two classes ( 3 ) which differ in 

 their whole construction: (1) the Gymnosperms, with naked seeds; 

 (2) the Angiosperms, with seeds enclosed in an ovary. 



The names of these classes indicate the nature of one of the most 

 important differences between them. THE CARPELS OF THE ANGIO- 

 SPERMS FORM A CLOSED CAVITY, THE OVARY, WITHIN WHICH THE 

 OVULES DEVELOP. SUCH AN OVARY IS WANTING IN THE GYMNO- 

 SPERMS, THE OVULES OF WHICH ARE BORNE FREELY EXPOSED ON 

 THE MACROSPOROPHYLLS OR CARPELS. 



The Gymnosperms are the phylogenetically older group. Their 

 construction is simpler and in the relations of their sexual generation 

 they connect directly with the heterosporous Archegoniatae ; they 

 might indeed be perhaps best treated as belonging to this group. 



The Angiosperms exhibit a much wider range in their morpho- 

 logical and anatomical structure. The course of their life-history 

 differs considerably from that of the Gymnosperms, and without the 

 intermediate links supplied by the latter group, the correspondence 

 with the life-history of the Archegoniatae would not be so clearly 

 recognisable. 



These conclusions are confirmed by the evidence afforded by 

 Palaeobotany. Gymnosperms or forms resembling them are found 

 along with what appear to be intermediate forms between the Gymno- 

 sperms and the Pteridophyta in the fossiliferous rocks of the Devonian, 

 Carboniferous, and Permian formations. The Angiosperms are, on 

 the other hand, first known from the Cretaceous formation. 



Table of the Classes and the most Important Orders 

 f and Families of Phanerogams 



Class I. Gymnospermae 



1. Cycadinac : Fam. Cycadaeeae. 



2. Ginkgoinac : Fam. Ginkgoaceae. 



3. Coniferae : Fam. Taxaceae, Pinaceae. 



4. Gnetinac : Fam. Gnetaceae. 



