INTRODUCTORY 25 



of individual organs, such as the periodic foldings of 

 the leaves of Leguminosae and the startling response to 

 contact shown by the " Sensitive Plant." These observa- 

 tions were among the earliest beginnings of the section of 

 plant physiology, sensitivity, that has attracted so much 

 attention in recent years. 



There is yet another department of botany to which 

 I must draw your attention, but it is one of comparatively 

 recent origin, and that is the study of fossil plants. It 

 dates from the early years of the nineteenth century, but 

 its importance was scarcely appreciated until more than 

 half-way through it, until in fact phylogenetic classifica- 

 tion demanded that the gaps in the system should, if 

 possible, be filled up Palaeobotany is thus essentially a 

 product of our own time. 



Theophrastus was acquainted with about 500 plants ; 

 some 2000 are described in the sixteenth- century herbals, 

 but at the present day the number has risen to something 

 like 300,000. This tremendous increase has been due to 

 the systematic search made by travellers in every quarter 

 of the globe, and the subject of plant distribution or 

 Geographical Botany now claims a volume to itself. 

 Detailed examination of these finds soon made it evident 

 that plants were not scattered haphazard over the earth's 

 surface ; that, on the contrary, their distribution was 

 governed by a complex series of factors ; that there were 

 centres of distribution, barriers and aids to migration, 

 and that the same plant type might appear in areas far 

 apart from each other although nowhere between. 

 Problems such as these linked themselves, on one side, 

 to the discoveries of palaeobotany and, on the other, to 

 plant physiology. In this latter relation a new line of 

 investigation has made its debut quite recently, which 

 you know under the name of Ecology, but which is after 

 all merely the grosser physiology of plant communities. 



You will thus realise that the field of botanical investi- 

 gation soon grew far too vast to be cultivated by one 



