1876.] MISCELLANEOUS BOTANICAL LETTERS. 515 



This fact and several others have led me to the suspicion 

 that the cause of variation must be in different substances 

 absorbed from the soil by these plants when their powers of 

 absorption are not interfered with by other plants with which 

 they grow mingled in a state of nature. Therefore my son 

 and I wish to grow plants in pots in soil entirely, or as nearly 

 entirely as is possible, destitute of all matter which plants 

 absorb, and then to give during several successive generations 

 to several plants of the same species as different solutions as 

 may be compatible with their life and health. And now, can 

 you advise me how to make soil approximately free of all the 

 substances which plants naturally absorb ? I suppose white 

 silver sand, sold for cleaning harness, &c., is nearly pure sili- 

 ca, but what am I to do for alumina ? Without some alumina 

 I imagine that it would be impossible to keep the soil damp 

 and fit for the growth of plants. I presume that clay washed 

 over and over again in water would still yield mineral matter 

 to the carbonic acid secreted by the roots. I should want a 

 good deal of soil, for it would be useless to experimentise 

 unless we could fill from twenty to thirty moderately sized 

 flower-pots every year. Can you suggest any plan ? for un- 

 less you can it would, I fear, be useless for us to commence 

 an attempt to discover whether variability depends at all on 

 matter absorbed from the soil. After obtaining the requisite 

 kind of soil, my notion is to water one set of plants with 

 nitrate of potassium, another set with nitrate of sodium, and 

 another with nitrate of lime, giving all as much phosphate of 

 ammonia as they seemed to support, for I wish the plants to 

 grow as luxuriantly as possible. The plants watered with 

 nitrate of Na and of Ca would require, I suppose, some K ; but 

 perhaps they would get what is absolutely necessary from such 

 soil as I should be forced to employ, and from the rain-water 

 collected in tanks. I could use hard water from a deep well 

 in the chalk, but then all the plants would get lime. If the 

 plants to which I give Nitrate of Na and of Ca would not 

 grow I might give them a little alum. 



I am well aware how very ignorant I am, and how crude 



