210 CONCLTJSIOK'S. 



occur under very different conditions of temperature. In the 

 present instance, the numerous species of cycads naturally suggest 

 conditions similar to those most favourable or essential to the 

 living representatives of the Cycadacece ; but we must remember 

 that the so-called cycadean fronds from Mesozoic rocks are nearly 

 always found apart from the stems and reproductive structures, 

 and we are still to a large extent in the dark as to the exact 

 nature and structure of these extinct cycadean plants. 



Looking at the Wealclen plants collectively, we notice a very 

 striking agreement with the flora of the underlying Jurassic 

 strata, and it would be difficult to point to any well-marked or 

 essential difference between the plant-life of the two periods. 

 The evidence of paloeobotany certainly favours the inclusion of 

 the "Wealden rocks in the Jurassic series. 



One of the most attractive and difficult problems which is 

 suggested to a botanist by such a flora as that of the Wealden 

 period, is the evolution of angiospermous plants. A reviewer has 

 happily expressed this in the following words l : "In the folds of 

 the Wealden we imagine the secret of the evolution of angiosperms 

 must be locked. It is as if we stood at the mouth of a great 

 river flowing from an unexplored interior, whose flotsam we 

 anxiously interrogated for clues as to the nature of the unknown 

 Hinterland ; yet nothing reaches us from beyond the coast-belt, 

 which we have already explored." Among the English species 

 there are none which can be regarded as the earliest angiosperms, 

 and we search in vain among the abundant samples of the Wealden 

 vegetation for any fragments of monocotyledonous or dicotyledonous 

 plants. In the Potomac beds of America, which include strata of 

 Jurassic and Lower Cretaceous age, we have several undoubted 

 angiospermous species ; and again, in the closely parallel series of 

 Portuguese rocks, dicotyledons and monocotyledons are fairly 

 abundant. The true Wealden vegetation would seem to have 

 been without any examples of the highest class of plants, and 

 may be looked upon as the last of the Mesozoic floras in which 

 the gymnosperms represented the limit of plant development. 

 One genus, however, carries us a few steps towards the next 

 stage in botanical evolution ; the inflorescence of Bennettites marks 



1 Nature, July 26, 1894, p. 294. 



