NUTRITION. THE MACHINERY. 19 



their character as absorbent organs. The one function 

 common to all roots is absorption. They may have other 

 offices to fulfil, and they have very varied forms ; but when 

 we come to consider the main function of the root, then we 

 find simplicity and relative uniformity of structure. The 

 thick woody limb of an elm root, as we see it exposed in a 

 hedgebank from which the soil has fallen, is no organ of 

 absorption ; the thick " bulbs " (so-called) of a turnip or a 

 beet are not organs of absorption ; neither are these latter, 

 any more than the tubers of potatoes, strictly speaking, 

 roots. Our best and truest conception of a root as an 

 organ of absorption is that of a single fibril or of a dense 

 mass of the finest fibrils root-branches no thicker than a 

 hair. These fibrils grow in length close to their tips, the 

 actual tip being covered with a thin extinguisher-like cap 

 of dead tissue the root-cap already mentioned, and which 

 serves as a shield to the softer tissues within. The 

 structure is of the simplest, merely layers of cells such as 

 before described, arranged in more or less longitudinal 

 ranks, the cells themselves delicate and thin-walled. The 

 fineness of the root-fibril, its growth near the tip, its 

 wonderful power of motion, are all well adapted to permit 

 of the fibril making its way between the particles of soil, 

 aiid extracting nourishment from the fluid surrounding 

 them. We have only to examine the root of a wheat plant, 

 or still better of a perennial pasture grass, to see how per- 

 fectly this is accomplished. Under such circumstances the 

 root-fibrils form a dense wig, as it were, of feeding threads 

 which occupy the soil so thoroughly that the soil is held 

 together by them. It is easy to see that although the 

 absorbent power of each thread is infinitesimal, yet in the 

 aggregate it must.be very large. Fine as they often are 



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