On the Structure of Roots. 473 



Is combined with an almost infinite amount of variation in the 

 details in which it is carried out. This circumstance gives rise 

 to two of the principal difficulties standing in the way of those 

 whose business it is to convey botanical knowledge to the un- 

 initiated. On one hand the great variety of forms and conditions 

 assumed by the parts of plants demand especial names, so that 

 they may be referred to in language sufficiently brief; on the 

 other hand the scientific definition of the organs of plants, that is 

 to say, the exact description of those characteristics which com- 

 pletely and at the same time exclusively distinguish each kind of 

 organ, is rendered a task requiring the utmost care ; and indeed, 

 if we look for absolute accuracy and the exclusion of all that 

 does not hold good universally, this becomes an impossibility. 



The attempt to define the Root is a fair example of these 

 difficulties. And that these questions are not mere " curiosities" 

 of science, but involve most important practical conclusions, is 

 nowhere better seen than in this kind of organ. A root is generally 

 known as that part of a plant which grows downwards into the 

 ground, and in the soil takes up the principal part of the nourish- 

 ment required by the rest of the structures. Speaking a little 

 more strictly we may say it is the descending portion of the same 

 central body {axis) which rises upwards in the form of the stem. 

 What we have spoken of above as a true root corresponds exactly 

 to this; when only adventitious roots exist, these form collectively 

 a descending bundle corresponding to the single axis of the tap- 

 root. 



When we turn our attention to practical facts, we soon arrive 

 at some questions which disturb our reliance in the above defini- 

 tions. The growing downwards of roots is a matter only of a 

 few inches, or a few feet, as the case may be, and they usually 

 then turn right and left under pain of destruction from the want 

 of those influences which are present in the upper layers of the 

 ground. Then as to function. The ivy on the garden wall 

 exliibits to us undoubted roots, in abundance, exerting no 

 nourishing office, but forming simple hold-fasts enabling the 

 stem to climb. 



The first of the exceptions just taken is by far the most im- 

 portant in a practical point of view. Those prolongations of the 

 originally descending root which run out horizontally in the 

 soil are very liable to be confounded with the stems of a vast 

 number of perennial herbaceous plants which grow in a similar 

 situation, beneath the surface of the soil, and acquire very much 

 of the external appearance of roots. So much is this the case, 

 that the forms of stem now referred to are commonly known in 

 gardens under the title of roots, their true nature as stems being 



VOL. XIX. 2 I 



