28 ROOTS 



Most water roots are destitute of root hairs, and absorb water 

 through the general epidermal surface of their younger portions. 



Aerial roots, like those shown in Fig. 13, are in many cases 

 provided with an external absorbent layer of spongy tissue, by 

 means of which they retain some of the water which trickles 

 down them during rains. This stored moisture they gradually 

 give up to the plant. 



36. Absorption of water by roots. Just how much water 

 some kinds of plants give off (and therefore absorb) per day 

 will be discussed when the uses of the leaf are studied. For 

 the present it is sufficient to state that even an annual plant 

 during its lifetime absorbs through the roots very many times 

 its own weight of water. Grasses have been known to take 

 in their weight of water in every twenty-four hours of warm, 

 dry weather. This absorption in most soil roots takes place 

 mainly through the root hairs. Their walls are extremely thin, 

 and have no holes or pores visible under even the highest 

 power of the microscope, yet the water of the soil penetrates 

 very rapidly to the interior of the root hairs. The soil water 

 brings with it all the substances which it can dissolve from the 

 earth about the plant; and the closeness with which the root 

 hairs cling to the particles of soil, as shown in Figs. 9 and 21, 

 must cause the water which is absorbed to contain more foreign 

 matter than underground water in general does, particularly 

 since the roots give off enough weak acid from their surface to 

 corrode the surface of stones which they enfold or cover. 



37. Substances required by the plant for nutrition. Ordinary 

 seed plants require for their nutrition ten of the chemical ele- 

 ments. ^ By far the greater part of the weight of the plant body 

 is usually due to compounds of carbon, hydrogen, oxygen, and 

 nitrogen. Besides these there are present the six elements, 

 sulphur, phosphorus, potassium, calcium, magnesium, and iron- 

 In ordinary green meadow grass there is about 80 per cent of 

 water and 20 per cent of dry matter. On drying the grass into 

 hay and then burning the latter, some 2 per cent of ash will remain, 



