For the Layman 



stand how plants grow, and to know what substances affect 

 their different kinds of growth. 



The understanding of substances belongs to the science of 

 chemistry, which has built up for itself that queer-looking 

 but exact-meaning terminology which I have already men- 

 tioned. The search for answers to the questions that centre 

 around the "how" of plant growth belongs to the science of 

 phytology, or plant physiology, and, in what follows, I have 

 written practically nothing under that head, because I am not 

 a plant physiologist, and don't understand the subject suffi- 

 ciently to discuss it in detail. However, some hints as to how 

 plants grow can be gained by observing the effect of different 

 substances on plants. The preparation and study of such sub- 

 stances is the job of the chemist. 



The substances that can be tested on any one kind of plant 

 fall into two groups: those which the plant can't produce 

 for itself, and those which it can. 



Plants cannot produce for themselves the mineral foods 

 they need, and which they normally obtain from the soil ; these 

 are such things as phosphates, nitrates, sihca, lime, potassium 

 salts, and so on. They also require carbon and water, which 

 they obtain from the air and the soil, and a plant growing 

 from seed has a reserve of food in the seed to start life with. 



Given these things, the plant grows. It gets bigger, but it 

 also becomes differentiated, that is, it acquires parts, such as 

 root, stem, and leaf. How does a seedHng "know" that it 

 must form a number of parts, all of them necessary, and of 

 several different types, and how does it arrange them into the 

 pattern which is recognizable by us, so that we can say "that 

 is a willow, and that is a tomato plant" ? 



There is possibly a clue in the activities of some of the sub- 

 stances which the plant produces for itself. A large number of 

 plant-products are structure-materials and reserve foodstuffs, 

 together forming the framework of the plant and its kitchen 

 and pantry. Other substances seem to have a directive or 

 husbanding effect. These latter are the natural growth- 

 regulating substances. They are also called hormones, or 

 "chemical messengers". 



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