CORRELATION OF TISSUES 67 



Sometimes it is the economic principle that is ignored : a good 

 illustration is provided by the wasteful expenditure of spermatozoids 

 or pollen-grains that takes place in connection with the process of 

 fertilisation. In other cases, again, the mechanical principle is 

 partially invalidated. The foliage of Musa paradisaica exhibits a 

 peculiarity which never fails to impress the visitor to the tropics : 

 the gigantic leaf-blades of this plant are torn, by the action of wind 

 and rain, into a series of strips which are held together solely by 

 the stout midrib. Anatomical investigation shows that the leaf of 

 Musa displays none of those arrangements for preventing laceration 

 of the leaf-margin which are otherwise of such general occurrence 

 in leaves. Exceptional cases of this kind are, however, by no means 

 necessarily irreconcilable with the general principle of efficiency ; on the 

 contrary, some special advantage usually accrues in such instances which 

 must be set off against the loss entailed by the exceptional feature. 

 The lacerated leaves of Musa are instructive from this point of view. 

 For not only do the torn edges of the strips heal rapidly, so that 

 no permanent damage results, but the fact that the strips subsequently 

 maintain a pendent position is actually advantageous, because the 

 leaf-surface is in this way withdrawn from the impact of the violent 

 tropical rains and from the scorching rays of the sun. 2 * 



Attention must next be directed to the so-called correlation of 

 tissues, a phenomenon which recurs in a great many different forms, 

 but which is often of somewhat obscure significance ; the term is 

 intended to comprise all those cases in which different tissues either 

 forming part of the same organ or else located in widely separated 

 regions of the plant-body exert a definite and constant influence 

 upon one another's activities. In the course of evolution those 

 interrelations of different tissues which react favourably upon their 

 physiological activity have gradually become fixed : in other words, 

 tissues have become adapted to one another, and have, as it were, 

 learnt to act in harmonious co-operation. 29 In the case of foliage 

 leaves, for example, the structure and arrangement of the photo- 

 synthetic tissue are intimately related to the course of the vascular 

 bundles, which are instrumental in conveying water with mineral 

 salts in solution to the photosynthetic cells, and in removing the 

 plastic materials manufactured by the latter. Water-tissues are 

 definitely orientated with reference to the photosynthetic cells, for 

 the use of which the store of water is in the first instance designed. 

 Even more remarkable is that type of correlation in which one tissue 

 or organ directly controls the activity of another. < Vi tain hydathodes, 

 for instance, start their otherwise autonomous water-excreting activity 

 only when the wood-vessels are water-logged and the hydrostatic 



