602 LECTURE XXXIV. 



were not impelled by various kinds of irritabilities — heliotropism, geotropism, sensi- 

 tiveness to contact, damp surfaces, &c. — to penetrate into the soil in order to collect 

 nutritive substances and to obtain a hold-fast, while the assimilating shoots and organs 

 of reproduction are impelled by irritabilities of other kinds to grow forth above 

 the substratum and receive the vivifying influence of light, in order to assimilate 

 and bear fruit? 



By this example we are led on to yet another point of great importance, 

 namely, the fact that by means of the same external influences reactions 

 exactly opposite in kind, though otherwise similar in nature, may be called 

 forth, so that we may in fact speak of a positive and a negative heliotropism for 

 instance ; and a similar contrast is noticeable in the case of other reactions also. 

 Of course the facts here coming into consideration can only be rendered clear in 

 succeeding lectures; but so much may be said even here, in anticipation of more 

 exact knowledge of them, that if the same external cause induces exactly opposite 

 effects in the organs, the explanation of this must simply be sought in the different 

 structure of the organs. If one organ when illuminated from one side becomes curved 

 so as to be concave on the side turned towards the source of light, while another 

 becomes convex on that side, the cause can only lie in the internal structure of the 

 organ. But it is just on such differences of structure that the great variety of 

 reactions which the most different plant-organs exhibit towards the same external 

 influences depends ; and, fundamentally, all that we term Biology — the modes of life 

 of organisms — depends upon the fact that different organs react differently towards 

 the same external influences, and these reactions differ not only qualitatively but also 

 quantitatively, the finest gradations existing in both cases. 



