EXTERNAL CONDITIONS ON PLANT LIFE. 165 



ever holding the plant invariable, while the external forces 

 are ever pushing it toward variation. In this lecture we 

 will disregard entirely the interesting phenomena which relate 

 to that internal force the manifestations of which we call 

 inheritance, and try to discover how much the plant world is 

 moulded and shaped by what is exterior to it. 



If you are inclined to think about all these differences in 

 the vegetation of the many regions through which you have 

 travelled you will soon see that where there is great change 

 in the appearance of the plants, in passing from one place to 

 another, there is also an equally marked difference in the 

 surroundings or soil or climate. You will soon find yourself 

 always looking for the -one when you have found an expression 

 of the other, i.e., if the plants give you a new flora, you seek 

 at once to find the cause in external conditions ; or, if you 

 have great external difference in surroundings and climate, you 

 expect this to be at once reflected in the varying vegetation 

 about you. 



Let us study a few plants a little more closely, and see 

 if we are able to determine on what this variation depends 

 and how rapidly it may take place. I shall assume at the 

 outset two sets of causes, both of which may be active in 

 bringing about variations in plants, the one wholly external, 

 such as light, heat, moisture and the like, and the other 

 internal, and concerned with the laws of inheritance, about 

 which we know so very little. 



Let us for our present purpose consider only the external 

 influencing conditions, for I believe that they are much more 

 potent and determinative than the laws of inheritance. They 

 may be enumerated as follows : 



i . The Water Supply. This is more important than any of 

 the following points, as it is intimately connected with the 

 very vital process of organizing materials for growth and 

 their transport throughout the plant. 



If the water supply is cramped and too little, we have 

 immediately before our minds the desert plants. 



If the water supply is too great, equally important changes 

 take place. 



