CORRELATIONS BETWEEN LEAVES AND ROOTS, ETC. 513 



and of the leaves) above all to bring those parts of the shoot which contain chloro- 

 phyll into a position favourable for their illumination, and the root-fibres into one 

 suitable for their activity. In many cases, as with twining plants and those \^hich 

 climb by means of tendrils, peculiar relations of organisation are developed in order 

 to bring the assimilating leaves into the light in spite of the fact that the shoot- 

 axes are thin and feeble : in the same way, the numerous leaves endowed with 

 so-called sleep-movements are always so organised and so react towards the light, 

 that they present their surfaces to it at the beginning of the day, and when the 

 light becomes too strong turn themselves so that the rays meet them at small 

 angles, and are thus rendered harmless. It is obvious that these irritabilities 

 also have meaning and purpose only in so far as it is important to bring the 

 assimilating organs containing chlorophyll into the most favourable position possible 

 for illumination. 



Even these correlations, which, as already mentioned, must have made their 

 appearance in the course of the progressive development of the vegetable kingdom, 

 may be at least in part subjected to experimental control, and indeed every-day 

 experiences in the cultivation of plants confirm the above conclusions. Here is one 

 example only. If a Tobacco-plant, or a Ricinus, or a Sunflower, &c., is allowed to 

 develope in the open and in good soil, there is formed in the course of 100-120 

 days a powerful stem, occasionally as thick as the arm, with numerous very large 

 leaves and an enormous root-system. If, on the other hand, we cultivate such 

 a plant in a flower-pot filled w-ith three litres or so of the best garden soil, the plant 

 in its pot standing in the open and being even watered two or three times daily with 

 proper solutions of nutritive substances, we obtain in 100-120 days a stem of the 

 thickness of a finger, and a total leaf-area which amounts to scarcely a fifth or a sixth 

 of that in the previous case, and, shortly, a small insignificant plant, in spite of 

 all the artificial supply of nutriment at the roots, and in spite of the bright 

 illumination of the leaves. But the cause exists in the limitation of the space for 

 the roots in the flower-pot : it is true any one unacquainted with the matter is 

 astounded at the apparently very large quantity of roots in the ball of earth Hfted out 

 of the pot, but as a matter of fact this is extremely small in comparison with the 

 rooting of a similar plant in the open land. Moreover the roots are unfavourably 

 situated : they are all crowded together, with their growing ends and the parts 

 specially adapted for the absorption of food materials close to the inside of the pot, 

 and there forming a dense felt work all round, which not only hinders further growth 

 and branching, but also completely excludes most of the roots from the soil in the 

 pot, and even makes impossible the proper employment of the nutritive solutions 

 with which they have been watered. The result of this inefficient growth of the 

 roots is a diminished supply of nutritive materials in the assimilating leaves, in 

 consequence of which their activity remains insignificant, and this again aflccts the 

 formation of wood in the stem. 



In this case the pernicious correlation proceeded from the roots ; but it may start 

 from the leaves also (even though the roots are fully developed in the earth), when 

 they are so feebly illuminated that assimilation, although not entirely prevented, is 

 reduced to a minimum. Together with a somewhat abnormal aspect of the plant in 

 other respects also, it is then especially found that the stem remains thin, forms but 



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