2 THE BIOLOGY OF FLOWERING PLANTS 



the maturing cell occupies much the largest fraction of its 

 volume. So we find that the greater portion of the 

 fresh weight of the plant consists of water ; an average 

 figure is 80 per cent., but in succulent organs such as 

 the leaf of a lettuce or the fruit of the strawberry, water 

 may account for more than nine-tenths of the total weight. 



(3) The subaerial organs of the plant, in particular the leaves, 

 constantly lose water vapour to the atmosphere ; they 

 transpire. This loss must be made good, and, normally, 

 a very large supply is necessary. It has been reckoned that 

 in the course of an i8-weeks' growing season a sunflower 

 loses 6 gallons of water in transpiration. The water 

 transpired during the growing season is several times the 

 total weight of the mature plant in herbaceous species, so 

 that the supply required to cover this loss accounts for much 

 the larger fraction of the water absorbed by the plant. 



(4) With the water supply from the soil the plant obtains 

 the elements potassium, magnesium, calcium, iron, 

 phosphorus, sulphur, and nitrogen, which are essential to 

 its development. 



On germination the first part of the seedling to leave 

 the seed coats is the radicle ; before the young shoot 

 appears above the soil the root may be several inches long 

 and have already started to branch. The immediate 

 demand for water is emphasised by this early making of 

 contact with the source of supply, the soil. 



§ 2. The Origin and Nature of Soil 



The soil covers the fertile land surface of the globe in 

 a layer which in general extends from a few inches to a few 

 feet in depth. Not only is it the normal medium of plant 

 growth, it is also a product of the vegetation it supports. 

 The commonest obvious distinction between the soil proper 

 and the sub-soil which underlies it is that the soil is darker 

 brown or almost black in colour because of the presence 

 of humus, the disintegrating and altered remains of dead 

 vegetation. 



