INTRODUCTION 13 



is undoubted, and it may be safely assumed that these differences 

 also involve differences in behaviour towards prejudicial external 

 influences. At present, however, we know very little in this 

 connection, and we can in the meantime only conjecture that 

 the explanation of the individual differences in the behaviour of 

 plants towards frost, drought, and even towards the attacks of 

 fungi will partly be found in such chemical differences. 



Even more striking are the cases where differences in the 

 physiological behaviour of plants serve as disease-inducing con- 

 ditions. It is well known how certain trees of the same wood 

 awake from their winter rest and become green at different 

 times, although in other respects they are perfectly similar. In 

 a young spruce plantation differences of two or even three weeks 

 may be easily perceived in the opening of the buds of different 

 individuals, and this must be accounted for, in most part, by 

 differences in the heat-requirements of the plants. It is evident 

 that early unfolding of the leaves implies a disposition for injury 

 by late frosts, but it may also become the chief stimulus to the 

 development of fungoid disease. If, for instance, the spruce-leaf- 

 rust (Chrysomyxa} is, in the spring, at the stage when its spores 

 are being shed, all those spruces whose buds have not begun 

 to elongate into shoots will remain entirely unaffected by the 

 fungus, which is only able to force its way into the delicate 

 leaves of young shoots. A disposition for this disease, therefore, 

 attaches to the individuals which begin to grow early. In 

 other years it may happen that those individuals which first 

 begin to grow are so far advanced in development when 

 Chrysomyxa sheds its spores that the leaves are already too old 

 to be susceptible to infection. In this case it is perhaps just 

 the late varieties that contract the disease. 



The observation that amongst the individuals of a plant 

 species there are always some whose requirements as regards 

 heat are less or more than those of others, and that these are 

 therefore disposed to suffer from cold to a greater or less extent, 

 and that, further, demands on the moisture of the air and other 

 factors of growth vary with the individual, has probably led to 

 importance being attached to the place of origin of the seeds 

 which we employ in cultural experiments with exotic species of 

 plants. We endeavour to obtain seeds from districts where, in 



