THE SPERMATOPHYTA : GENERAL INTRODUCTION 643 



This tree growth shows the descendants of the Pteropsida again doing 

 something which the Lycopsida had already done at a lower evolutionary 

 level in the Lepidodendrons. The parallelism in the two groups is very 

 interesting and raises the question of a connection between strobilus forma- 

 tion on the one hand and tree growth on the other. Present evidence is, 

 however, against the assumption of an evolutionary linkage between them. 

 The differences of structure are in fact profound, and it would seem that 

 we are here presented with an instance of evolutionary convergence. The 

 two alliances, having been forced, so to speak, along the same path by funda- 

 mental factors of the environment, were obliged to solve the same problems 

 independently by closely similar means. 



Evolution of the Angiospermic Flower 



Above the Gymnosperm level the next stage in evolution has been the 

 enclosure of the seed within a covering, the carpel, formed possibly from the 

 appendage which bears it. The embryo thus acquires yet another protection, 

 one which is capable of great morphological variation and hence is very useful 

 in the evolutionary sense. This brings us to the highest group, the Angio- 

 spermae or plants with seeds enclosed in a vessel (Gr. aggeion), which 

 begins as a carpel and later develops into a fruit. So vast and multifarious 

 is this group that a large portion of the rest of this book will be needed in 

 which to deal with it. 



The Angiosperms are the dominant form of plant life over the greater 

 part of the world, and they include almost all the plants with which we are 

 familiar in field, woodland or garden. They have shown unrivalled plasticity 

 in adapting themselves to every form of environment, from deep lakes to 

 deserts, and from the bed of the sea to high mountain peaks. The same 

 power of variation is shown in the production of species numbering hundreds 

 of thousands, which display almost every conceivable variety of plant form. 



This immense success is due to a combination of many advantages, but 

 we may perhaps point to three which have certainly played a great part : 

 (i) insect pollination, (2) the development of many devices for seed dispersal 

 and (3) the herbaceous habit. 



It is a far cry from the Pine Cone to the Rose, and we may well inquire 

 what connection can be traced between them. What agency could bring 

 about the transformation of a hard, woody, uninviting strobilus into the soft, 

 tinted and perfumed Queen of Flowers ? The answer is insect pollination 

 and the preference of certain insects for just those qualities which distinguish 

 the angiospermic flower from the cone. It is not for us that the Rose and 

 Violet bloom, it is for the bees ! 



Recognizable Angiosperms first appeared at the end of the Jurassic 

 period, and in a relatively short time afterwards we find many fossil types 

 which are apparently similar to genera still living. It is a dramatically rapid 

 step in evolution, which we can only understand if we suppose that there 

 must have been some entirely new and powerful factor at work 



