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I 



VARIOUS CAUSES OF GROWTH. 775 



its original condition. The sensitiveness of the leaves of Mimosa does not there- 

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

 tendrils 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 connected with them 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 organising 

 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 

 and non*growing parts (for instance a bulb suspended and putting out leaves in the air), 

 is not nourished as such from without. The growth of certain parts is therefore no indi- 

 cation of nutrition of the whofe. Still less necessary is the connection between growth 

 and nutrition from without ; the special organs of nutrition, the green leaves, do not 

 grow after they are mature, although they carry on the process of nutrition. The two 

 processes may coincide both in place and time, i.e. in the same cell; but may also be 

 separated in both space and time ; and this is indeed usually the case, as has been suffi- 

 ciently shown in Sect. 5. 



Sect. 12. Various Causes of Growth. Growth, like all vital processes, takes 

 place only when certain favourable external conditions coexist. These are the presence 



