114 THE LIFE OF THE PLANT 



readily, so that it has no need of a particularly large 

 surface. 



We have said more than once that a root probably 

 obtains food also from the hard particles of the soil ; 

 but how is this to be explained ? The surface layer of 

 the root with its hairs consists of small cells, which 

 have, of course, no apertures in their walls. The 

 particles of the soil can come into very close contact 

 with the root hairs, as is shown in fig. 33 D, but never 

 pierce their walls. How can we reconcile these con- 

 tradictory facts, that solid bodies serve as food to the 

 root and yet do not pass through its cell-walls ? In 

 order to explain this apparent contradiction, we have 

 recourse to the following experiment. A glass jar is 

 filled up to the very top with water, and then covered 

 with a bladder. The external surface of the bladder is 

 carefully wiped with blotting-paper, so that it appears 

 to be quite dry. We scatter some chalk powder on the 

 surface. Chalk is a solid body ; the bladder does not 

 contain any apertures ; and yet we soon find that the 

 chalk disappears from the surface of the bladder, passes 

 through it, and appears in the solution inside the jar. 

 We need not wait until all the chalk disappears from the 

 surface of the bladder, for we possess very delicate 

 reagents for detecting the presence of lime in water. 

 This colourless liquid (ammonium oxalate) , for example, 

 has the property of forming a white precipitate with 

 soluble lime salts. I pour some of it into the water 

 taken out of the jar before the experiment ; there is 

 no precipitate. I take some of the water out of the jar 

 some time after the chalk has been lying on the bladder ; 

 I pour into it some of the reagent, and get an abundant 

 white precipitate, which means that the water already 

 contains lime ; that is, part of the chalk has passed 

 through the bladder. This experiment, which rather 

 surprises us at first, can be very simply explained. 

 However carefully we wipe the bladder with the blotting- 



