694 MECHANICAL LAWS OF GROWTH. 



plant bein^ altered permanently and to an amount varying with the species by the 

 operation of the turgescence. It is again the result of permanent and specific 

 change of organisation when a tendril, in consequence of the light pressure of the 

 body to which it clings, lengthens less on the side in contact, more on the opposite 

 exposed side ; the curvature thus caused does not disappear if the pressure has 

 lasted long enough ; the whole phenomenon is therefore one of growth. When, on 

 the contrary, the motile organ of a Mimosa-leaf bends downwards in consequence 

 of irritation, and afterwards again bends upwards, this is, it is true, caused by the 

 peculiar organisation of the plant; but the movement induces no change in the 

 organisation itself, and its effects are not permanent, the leaf soon returning to its 

 original condition. The sensitiveness of the leaves of Mimosa does not therefore 

 depend on a change of growth caused by the irritation ; while the power of ten- 

 drils to curl round supports depends, it is true, on sensitiveness, but of such a 

 character as to cause a change in the processes of growth. 



If increase in volume is included in the idea of growth, as is the case in 

 ordinary language, the rigorously scientific use of the word would require special 

 care ; for if we simply say that a plant or a part of a plant of considerable size 

 grows, this may be accompanied actually by a decrease of the whole volume. Thus, 

 for example, when bulbs sprout or seeds germinate in the air, the whole does not 

 grow, but only the younger parts develope at the expense of the older, which in 

 addition give off aqueous vapour and carbon dioxide. It is therefore necessary to 

 distinguish accurately the growing parts from those which do not grow. 



There are however changes of form in the parts of plants which are not asso- 

 ciated with increase, and which may even be attended with decrease in volume, but 

 which nevertheless depend on a permanent and irreversible change of organisation. 

 Thus, for instance, the pith, after removal from the internodes, increases in length 

 for days even while it loses water by evaporation in air that is not saturated. It 

 would scarcely seem convenient to exclude these and similar phenomena from the 

 idea of growth; and it is therefore necessary to distinguish between growth with 

 and growth without increase in volume ; in the latter case growth consists in a mere 

 change of form which again depends on an alteration of position of the smallest 

 particles. Every case of increase in volume of a grain of starch or of a cell must 

 not be regarded as growth, inasmuch as it may be caused by absorption, and 

 may be reversed by loss of water ; nor is it necessary that growth in a single cell 

 should be associated with increase in volume, since particular parts of the cell may 

 furnish material for the increase of other parts. In this case the cell considered as a 

 whole only changes its form ; and if this change is caused by internal organic forces, 

 it must be considered as a kind of growth. Those changes in the form and volume 

 of cells must, on the other hand, be excluded from the idea of growth which occur 

 only occasionally and admit of being completely reversed, as is the case with the 

 contractile organs of sensitive and periodically motile leaves. 



An error which is constantly made by those who are unacquainted with physiology 

 is to confuse the ideas Growth and Nutrition, or to consider them identical. It is no 

 doubt true that all growth must be associated with the conveyance of food-materials 

 to the growing parts ; but these food-materials are usually withdrawn from older parts 

 where they were previously inactive ; the whole organism, consisting of both growing 



