110 BRITISH EOCENE FLORA. 



Hepaticae are similar in growth, and, with the Mosses, present a stationary group which 

 has elaborated a special kind of differentiation, but, since evolution has acted exclusively 

 on the first generation, in a direction limited by biologic conditions. 



Some of the vascular Cryptogams, as the Eerns, Equisetacese, and Ophioglossese, present 

 a further stage of development. Their life-history, as in the Mosses, is divided into two 

 generations which are extremely different, not only morphologically but physiologically. 

 Their spores give rise to a cellular thallus or " prothallium," never differentiated as in the 

 higher Mosses into a stem and leaf, but producing sexual organs, namely, archegonia and 

 antheridia. The sporogone, or young plant of the second generation resulting from the 

 fertilisation of the archegonium, instead of remaining a mere fruit-like appendage of the 

 sexual plant, as in the Mosses, is vigorous and independent, and speedily supplants the 

 ephemeral parent prothallium. As soon as free it takes root and finds its own nourish- 

 ment ; its tissues become extremely diversified, and fibres and vessels, histological elements 

 previously unknown, are developed, and plants known as Eerns and Horse-tails result. 

 In some of these the spore-producing generation, or sporogones, attain great age and very 

 considerable dimensions, as in Tree-ferns ; but on the leaves of all alike spores are born 

 whose germination produces a new generation of humble cellular prothallia. The vast 

 preponderance of this new vegetative system, the sporogone, is already manifest, and in 

 the next grade of plants we shall see that the independent sexual generation is still more 

 effaced. 



In the Rhizocarpeae, Selaginellese, and Isoeteae, a yet further step towards the evolution 

 of the highest plants, the Angiosperms, is made, for the separation of the two sexes is 

 foreshadowed in the two kinds of spores which they produce. 



Of these the macrospores are female, for they develop a prothallium bearing exclu- 

 sively female organs ; and the microspores male, inasmuch as their smaller prothallia only 

 bear male antheridia. In the Rhizocarps distinct progress is made towards the suppres- 

 sion of the sexual generation, for the female prothallium is so reduced that the sporogone 

 appears to be almost directly disengaged from the macrospore. Merely a small appendage 

 remains formed and nourished in the macrospore, and with only a very small external 

 development. In the sporogone of the Rhizocarp a " sporocarp," or kind of fruit, 

 appears, which is formed through the differentiation of some of the fronds, and contains 

 both micro- and macrospores enclosed in sporangia; in Marsilia this fruit reaches the 

 highest point of complexity to be met with in existing Cryptogams. The Selaginellas 

 and Isoetes present an equal development, for the prothallium is retained within the 

 macrospore itself as a mass of cell-tissue, in which true archegonia appear, destined 

 to receive the antherozoids on becoming exposed by the rupture of the cell-wall of the 

 spore. In the microspores the male prothallium is a wholly rudimentary organ reduced 

 to a single vegetative cell without function, and apparently a useless appendicle to the 

 antherozoid-producing cells which accompany it. To complete the metamorphosis to 

 Phanerogams the only further step required is that the macrospores should be fertilised 



