184 
Walter Stiles and Ingvar Jorgensen. 
cut in various ways, between the sections of which the plant is 
supported in cotton or asbestos wool. The essential point is that 
the support given to the plant should be sufficient to keep it in place 
and at the same time not so great that the part of the plant 
surrounded by the cork is compressed so as to damage it or hinder 
its growth in width. 
Means are also adopted to prevent the growth of fungi on the 
cork, as for instance by using new corks and immersing them 
before use in melted paraffin wax. Some workers use slabs of wax 
instead of cork. Growth of fungi on the experimental plant is 
prevented by keeping the upper surface of the nutrient solution out of 
contact with the cork or wool. Difficulties arising from this cause 
are probably greater in some cases than in others. Our experience 
is that a great deal too much has been made of them, and in many 
cases no trouble is likely to arise from this cause. Great care 
is usually taken to keep attached seeds out of the culture solution, 
but it is not clear whether this precaution is more necessary than 
many others which are completely neglected. 
4. The nutrient solution. Examination of the literature 
reveals an enormous number of nutrient solutions which have been 
recommended by various investigators for general use in water 
culture experiments. These differ much among themselves both as 
to the total concentration of the solution and the relative proportions 
of the constituents. Attempts have been made to examine the 
effect of altering both these factors, with the most divergent results. 
Thus, some workers have found the concentration of the solution to 
have little or no effect on the growth of plants, others, on the 
other hand, have found the reverse and have stated a definite 
strength of solution as the optimum for plant growth. Naturally 
enough different observers find different optima. The explanation 
of this divergence in results is to be found in the failure to realise 
the complex interaction of a number of factors left unconsidered, as 
well as in the assumption that the culture solution is unaffected by 
the presence of the plant growing in it. 
Similar considerations hold in regard to the influence of vary¬ 
ing the relative proportions of the various constituents. For some 
time now the idea of physiological balance between the constituents 
of the nutrient solution has been gaining ground. The greatest 
rate of growth is supposed to result when the constituents are 
present in a certain definite ratio. It is again significant that the 
ratio found by various observers is different; it varies also for 
different concentrations and different species. 
