INTRODUCTION. 



ALL that was known in the i6th and at the beginning of the 

 iyth centuries of the phenomena of life in plants was scarcely 

 more than had been learnt in the earliest times of human 

 civilisation from agriculture, gardening, and other practical 

 dealing with plants. It was known, for instance, that the 

 roots serve to fix plants in the soil and to supply them with 

 food ; that certain kinds of manure, such as ashes and, under 

 certain conditions, salt, strengthen vegetation ; that buds 

 develope into shoots ; and that the blossom precedes the 

 production of seeds and fruits. These and a variety of minor 

 physiological phenomena were disclosed by the art of garden- 

 ing. On the other hand, the physiological importance of 

 leaves in the nourishment of plants was quite unknown, nor 

 can we discover more than a very indistinct perception of the 

 connection between the stamens and the production of fruitful 

 seeds. That the food-material taken up from the soil must 

 move inside the plant in order to nourish the upper parts was 

 an obvious conclusion, which it was attempted to explain 

 by comparing it with the movement of the blood in animals. 

 Writers on the subject up to the end of the iyth century 

 make very slight mention of the influence of light and warmth 

 on the sustentation and growth of plants, though doubtless the 

 operation of these agencies in the cultivation of plants, as in 

 other matters, must have been early recognised. 



So scanty was the stock of knowledge which the founders of 

 vegetable physiology in the latter half of the zyth century 

 found ready to their hand. While the physiological signi- 

 ficance of the different organs of the human body and of most 



