THE LIFE AND GROWTH OF SEEDLINGS 23 



The following experiment illustrates this process. If the mouth 

 of a thistle-funnel be covered with a tightly stretched piece of 

 pig's bladder, and if the funnel, half -filled with a solution of sugar 

 and water, be placed in a glass vessel containing water, some of 

 the sugar and water will pass into the water and vice versa. The 

 cell-walls of the root-hairs may be compared with the bladder ; 

 they allow of the passage of water, and substances dissolved in it, 

 into the cell. Soil water necessarily contains those salts which 

 are present in any given soil. Now, whilst water can enter and 

 leave the cell freely, the salts contained in it are more or less 

 imprisoned in the cell protoplasm, and have the effect of stretching 

 the cell, producing the turgid condition on which growth depends. 



It is clear that soils must have the elements necessary for the 

 food of plants, if they are to thrive. The essential elements may 

 be ascertained in one of two ways : either by analysing any given 

 plant and thus ascertaining its chemical composition ; or by 

 growing plants in solutions containing certain salts, and seeing 

 how they thrive. The first method is largely adopted in agri- 

 culture. The exact chemical composition of the crop to be 

 planted in a particular field is ascertained, then the soil is analysed, 

 and any deficiency of an essential element is made good by appro- 

 priate manure. It is hardly too much to say that right manuring 

 makes or mars a crop, and may make all the difference to the 

 profits of the farmer. A good deal may be ascertained by experi- 

 mental plots. The individual plot should be about 20 square 

 yards ; the plant in question may be grown in a plot without 

 manure, then in one with sulphate of potash, in a third with 

 nitrate of soda, in a fourth with superphosphates, and so on. 

 The plants obtained from each plot should be weighed under 

 similar conditions. In this way the most appropriate manure 

 may be ascertained. The second method of water cultures is 

 of special value in laboratory work. Great care has to be taken 

 in preparing the water culture solution ; it is very difficult 

 to get it absolutely pure, and also to prevent the seedlings 

 " damping off." Any practical book on the physiology of plants 

 gives methods of setting up water cultures, and experience soon 

 shows which are the best seedlings to take. 



Wallflowers and buckwheat, also pea, bean, and maize, give 



