Lecture XXIV. 211 



The increase in the proportional size and in the complexity of 

 structure of the sporophyte has resulted in giving to the pine 

 a vastly more efficient equipment for life on the land. The 

 mechanical arrangement of its parts and the details of its structure, 

 internal and external, enable it to carry on its life under the 

 ordinary conditions on the land even when exposed to dry atmos- 

 pheric conditions and a relatively small supply of water in the 

 soil. 



This complete equipment of the sporophyte for life on the land 

 would be useless if, as in the case of the Fern and Mosses, the gameto- 

 phyte required aquatic conditions for the passage of the sperm to 

 the ovum. The ability of the male gametophyte to live as a 

 parasite on the nucellus and the retention of the female gameto- 

 phyte within the ovule enable this generation to dispense with 

 aquatic conditions while the sperm cells are transmitted through 

 the tissues of the sporophyte in the pollen-tube to the ovum. The 

 possibility of this state of things is led up to, as we have seen, in 

 a species of Selaginella where the microspores fall on to the 

 megaspores before they are shed from the megasporangium, and 

 the fertilisation of the female gametophyte is effected there. In 

 the heterosporous Pine the connection of female gametophyte and 

 the megasporangium is more complete and more prolonged. 

 There the inner tissues of the ovule or megasporangium remain in 

 nutritive contact with the megaspore (embryo-sac) throughout its 

 germination, its development and sexual maturity. Even later 

 during the development and early differentiation of the embryo 

 this connection is maintained and is continued, till both the 

 gametophyte and the tissues of the megasporangium are finally 

 disintegrated by the germination of the seed. This retention of 

 the female gametophyte in the megasporangium and the continued 

 nutritive connection between the two till the independence of the 

 embryo is established constitute the really important characteristics 

 which distinguish Seed-Plants from the present ferns. It is a point 

 of the highest interest that this distinction was not always maintained. 

 Numbers of fossil heterosporous fern-like plants have been found 

 having these characteristics. They belong to an ancient flora and 

 date as far back in the Earth's history as Palaeozoic time. These 

 plants have died out in more recent periods, and have left a gap 

 separating the present ferns and the Seed-Plants. 



From another point of view the seed is a most remarkable 

 structure. In its tissues are found an epitome of the whole life- 

 history of the seed-plant. Its coat and remnants of the nucellus 

 are portions of a sporophyte : the embryo-sac and endosperm 



14* 



