610 



THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



sexual cells of Spirogyra takes place (c/. vol. i. Plate I, fig. 1), or for this spermato- 

 plasm to fuse with the gametes of Ulothrix (cf. p. 49), as for the gametes of 

 Ulothrix to enter and fuse with the egg-cell of a Vaucheria; no new group of 

 plants could have arisen in this way. We may conclude then that plants belonging 

 to groups with a marked diversity in their sexual characters have not arisen from 

 one another, but belong to stems which have co-existed as distinct types from 

 the first. 



Nor must we omit to notice the observations which have been made in regard to 

 symbiosis and the inter-relations of green and non-green plants. It has already 

 been pointed out (vol. i. pp. 268, 264) how that the continuation of plant-life, and 

 indeed all life, is dependent on the activity which saprophytic plants exhibit in the 

 decomposition of dead organisms. Green plants could not exist independent of 

 colourless saprophytes, nor these latter without green plants. This must ever have 



been the case, and 

 it must so remain. 

 We may then draw 

 another conclusion, 

 viz. that those colour- 

 less plants which by 

 their activity bring 

 about the decomposi- 

 tion of gi'een plants 

 cannot have arisen 

 from green plants, 

 but that from the beginning they have belonged to a distinct stem. 



It is to Paleeontology that we must look for the most trustworthy solution 

 of the question as to whether numerous plant-stems have existed side by side from 

 the first, or whether the groups which at present co-exist have in process of time 

 branched forth from a single stem. Were it a fact that those forms which show 

 a far-reaching division of labour, and a complex structure of organs, which we term 

 " higher plants ", have arisen from others of very simple mode of life and possessing 

 a simple structure and which are known as " lower plants ", then should we expect 

 the earth to have been covered formerly by lower plants alone. And then, fol- 

 lowing this epoch, would have come a time when plants would have existed which 

 might have served as the starting-points of the later-appearing distinct groups. We 

 should expect to find in those strata of the earth's crust regarded by geologists 

 as the oldest of all nothing but the remains of very simple Thallophytes, then, 

 following these, Wracks, Red Sea-weeds, and Lichens, and after these Stoneworts 

 (Chara), Mosses, or some other type of plant which, having given rise respectively 

 to Stoneworts and Mosses, has, after this difierentiation, become extinct. 



From the graphite, which is looked upon as the oldest trace of vegetable life 

 on the earth, unfortunately we obtain no conclusive evidence on this matter. From 

 its presence on slate mountains together with crystalline limestone and pyrites 



Fig. 365.— Spirophy ton from the Upper Devonian. 



