66 



FOOD FEOM THE SOIL 



tions ; and they are the principal means by which plant - 

 food is absorbed from the soil, although the surfaces of 

 the rootlets themselves do their part. Water-plants do 

 not need an abundant system of root -hairs, and such 

 plants depend largely on their rootlets. 



139. OSMOSIS. In order to understand how the water 

 enters the root- hair, it is necessary that we study the 



physical process known as os- 

 mosis. A salt solution sepa- 

 rated by a membrane from 

 water absorbs some of the water 

 and increases its own volume. 

 First dissolve one ounce of 

 saltpeter, which we may use as 

 a fertilizer solution, in one 

 pint of water, calling this so- 

 lution No. I. For use in ex- 

 periments later on, also dis- 

 solve a piece of saltpeter not 

 larger than a peach pit (about 

 one -seventh ounce) in about 

 one gallon of water, calling this solution No. II. Now fill 

 the tube, C in Fig. 106, almost full of the strong solution 

 I, and tie a piece of animal membrane (hog's bladder is 

 excellent for this purpose) over the large mouth. A small 

 funnel, with a long stem, may be used if one cannot obtain 

 a tube like C. Then sink the tube, bladder -part down- 

 wards, into a large bottle, A, of water until the level of 

 liquid in the tube stands at the same height as that in the 

 bottle. The tube may be readily secured in this position 

 by passing it through a hole in the cork of the bottle. 

 In a short time, we notice that the liquid in N begins to 

 rise, and in an hour or so it stands as at F, say. This 

 is an important result. The facts are that the liquids 

 tend to diffuse, but the strong solution in N cannot pass 



105. Cross section of root, enlarged, 

 showing root-hairs. 



