362 SMITH : APPLICATION OF THE THEORY OF 



the an was saturated or almost so with moisture, and also the 

 etion of drops of water from the growing culms was a cer- 

 tain sign of superfluity of water supply. Such conditions are 

 quite exceptional. They are nowhere approached in Darwin's 

 observations and probably will be seldom possible in any series 

 of measurements. 



Reinke (14) deals with the question of irregularities of growth 

 and their dependence on internal or external conditions. He 

 is able by more refined methods to attribute some of the 

 " Stossweise Aenderungen " of Sachs to small changes in the 

 external conditions, but finds others which are really sponta- 

 neous changes independent of external conditions. He has one 

 ease of great regularity however in which the moisture of the 

 air is the limiting factor throughout. This is the case of the 

 growth in thickness of the stem of Datura Stramonium. His 

 conclusion presents one very interesting point for our theory, 

 namely, that his plants became acclimatised to a smaller supply 

 of moisture which at first caused a strong decrease in growth 

 rate. This fact cannot be reconciled with the idea of limiting 

 factors at all. for if the first effect, i.e., decrease of growth is due 

 to the water supply being insufficient for the faster growth, 

 then we cannot expect any increase until the water supply is 

 in' leased. It is possible however that changes of turgidity 

 without any growth change would have considerable effect on 

 the thickness of the stem and, as Lock (12) has pointed out, the 

 researches of Kraus (9) have shown a periodic interchange of 

 water between the wood and the cortex of certain stems which 

 is accompanied by a change of size of the stem. The whole 

 i hange may not therefore be due to growl h alone. 



It docs not -'■'■in unlikely therefore taking aU the results into 

 consideration thai the idea of limiting factors may supply a 

 gene i a I hasi "i explanation lor a considerable number of cases 

 of growth, anion- which there hitherto appears to have l>een 

 Little agreement. It is desirable that numerous measurement 

 should be taken of the growth of the different parte of plants 



