37 



water), N/5, N/io, N/20. In each experiment with barley 120 plants 

 were grown in units of ten : — 



I. — All concentrations (N, N/5, N/io, N/20), the solutions being 

 changed regularly every four days. 



2. — All concentrations, the solutions being changed once, half way 

 through the experiment. 



3. — All concentrations, the solutions never being changed. 



An examination of the figures and curves of dry weight shows that 

 in all cases there is a steady decrease in the dry weights of the plants 

 as the strength of the nutrient solution becomes less. This decrease 

 in weight is very considerable and always outside the limits of experi- 

 mental error. The results run in the same direction in all the experi- 

 ments, the differences being accentuated in the sets grown later in the 

 year, when growth is more rapid. The falling off in dry weight, as the 

 concentration of the solution falls from N to N/5, is far less when the 

 solutions are changed frequently, and in some cases there is very little 

 difference in the two cases. This suggests that the N/5 solution might 

 be as favourable to growth as the N solution if it were sufficiently 

 frequently renewed. There are, however, indications that toxic effects 

 would set in under these circumstances in the N solution, as some of 

 the constituents might be present in so great a quantity as actually to 

 check plant growth. In the N/5 solution, on the other hand, this action 

 is less likely, and the plants could continue to make full use of the food 

 salts, thus approximating in growth to those in the N solution. If this 

 supposition be correct, it is not true to say that the plant is indifferent 

 to the variation in the strength of these two solutions, but that up to 

 a certain limit it responds to increased strength by increased growth. 



With the highest concentration, however, another factor, toxic 

 action, comes into play, counterbalancing the increased growth 

 and reducing it to the level attained with the lower concentration. 



XXIX. " Recolonisation of Cultivated Land allowed to revert to 

 Natural Conditions." Winifred E. Brenchley and 

 Helen Adam. Journal of Ecology, 1915. 3, 193-210. 



Broadbalk and Geescroft Wildernesses were originally under arable 

 cultivation, but they have been left untouched for many years and 

 allowed to recolonise themselves. Careful observations of the herbage 

 have been made from time to time, and for at least twelve months in 

 1914-1915 the changes in each flora were noted every month. Broad- 

 balk wilderness is in a relatively dry situation and the soil contains 

 calcium carbonate ; Geescroft is wetter and there is practically no 

 calcium carbonate present. 



Broadba,lk wilderness consists of two distinct parts : — 



1. an area which has been left untouched and has developed 

 into an oak-hazel wood ; 



2. an area from which the trees and shrubs have been grubbed 

 out at regular intervals and is now colonised by a great 

 variety of herbs,* notably Arrhenatherum avenaceum with a 

 good deal of Centaurea nigra, Poa trivialis, Agrostis sp., 

 Heracleum spondylium and many others. 



• Using the word in its strict botanical sense, to comprise all plants except shrubs and trees. 



