480 BOTANY part ii 



acteristic 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 

 generation the single number of chromosomes. The gametophyte is 

 haploid, the sporophyte diploid (cf. p. 95). 



The Spermaphyta are divided into two classes (^) 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. 



Morphologry and Oecologry of the Phanerogramic Flower 

 and of its Sexual Generation 



The Phanerogamic Flower (^) 



The distinction between the gymnospermic and angiospermic 

 plants is briefly and well expressed by the statement that the seeds 

 of the Gymnosperms are exposed while those of the Angiosperms are 

 enclosed. Only an accurate examination of the floral features, especi- 

 ally of the structure, equipment, and development of the sexual genera- 

 tion in the two cases will enable us to form a correct judgment on 

 the differences between these two great classes of Phanerogams. 



The flowers of the Gymnosperms are all unisexual and diclinous. 



