CORRELATIONS 



335 



further growth, and the way in which they grow depends on the stock. The 

 external form is thus correlated with the life duration, which in one case lasts 

 one year (i.e. the inflorescence), in the other two years. 



Of equal interest is an experiment of Lindemuth (1901) on potatoes. The 

 potato may be grafted successfully on Datura, for certain buds of the stock 

 develop directly into horizontally placed aerial stola. In the course of the 

 formation of these stola the disposition of the plant makes itself felt in the 

 production of reserve stores which do not arise on the Datura stock. The stock, 

 however, encourages an active vegetative growth, doubtless owing to its great 

 capacity for forming roots, and the stola become not tubers but leafy shoots. 

 If, however, the scion be grafted on such a stock of Capsicum annuum, 

 which induces only feeble growth, the buds in question become tubers without 

 any formation of stola. 



We have spent too long over specific examples of correlations where 

 attention has been paid more to the larger members (roots and shoots) and less 

 to the tissues and cells. These latter also present us with abundant illustrations 

 of correlation, although they have often not been experimentally established. 

 To take one case only, we may recall the 

 apposition of pits in cell-walls as an ex- 

 ample of perfect correlation. 



We have now in conclusion to 

 summarize our conception of the opera- 

 tion of correlative influences ? In cer- 

 tain simple cases we may explain them 

 by a reference to nutritive factors. If 

 out of many buds only some develop we 

 must not assume that the non-develop- 

 ment of the others is directly due to a de- 

 ficiency in nutriment, since there is as 

 much nutrient there as is necessary to 

 permit all the buds to begin to grow. But 

 this would be extremely disadvantageous 

 for the plant. Obviously there is here an 

 adaptation of such a kind that no growth 

 begins when nutrition is feeble, and 

 the amount of nutritive material must 

 act as a stimulant in some way not known. In complicated cases of corre- 

 lation, such as the connexion between leaf and leaf -spur mentioned above (p. 330), 

 we cannot avoid assuming the existence of complex stimuli, although we are 

 unable to determine their nature. We must look more closely into this case and 

 investigate how the stimulus may be transferred from the leaf to the next 

 lower internode. It is easily shown, in the first place, that the development 

 of the vascular bundle is quite independent of assimilation and the nutritive 

 processes connected with it, since the experiment may be carried out equally 

 well in light and darkness. We might further believe that the functional 

 activity of the vascular bundle on the side of the leaf — the passage of water 

 through the vessels and of plastic organic materials through the phloem — 

 provides the stimulus from which follows the secondary thickening (De 

 Vries, 1891). Such ' functional stimuli ' doubtless play a certain part in 

 ontogenesis, but in the cases already cited they are not decisive factors at all 

 events. We may, as shown in Fig. 102, split the epicotyl of Phaseolus longi- 

 tudinally and then cut through transversely the part of the vascular system 

 supplying the leaf left attached ; the result is that the leaf-trace bundles passing 

 backwards from the leaf develop quite as well as in the previous experiment 

 although the nutritive stream from the cotyledons no longer flows in them nor 



/// 



Fig. 101. Diagram of a plant of Phaseolus^ on 

 which only one primary leaf is allowed to develop. 

 /-///, transverse sections at successive elevations; 

 ^, vascular bundles. 



