16,6 On the Natural Histm-y of Vegetables. 



this function, they must be extremely susceptible, and conse- 

 quently of the most delicate structure. This is obvious enough 

 to the naked eye, but much more so when assisted by the micro- 

 scope ; such sensitive organs are not fitted to bear the influence 

 of light, unless submersed in water, or in a very humid state 

 of the air, as we often witness in a hot-bed, or other confined 

 place. Darkness, therefore, is necessary for the natural action 

 of fibres ; and this, together with a certain degree of heat and 

 humidity, accounts for their prone direction in ordinary cases, 

 or for their taking any other direction if unnaturally situated 

 by art or accident. 



When the root of a seedling tree has descended an inch or 

 two into the soil, it is in shape a lengthened cone ; its exterior 

 becomes of a darker colour, and beset with transverse indented 

 marks, or minute tubercles ; from the former issue fibres, 

 which gradually increase in length, and become branching 

 radicles, which in time take all the appearance and offices of 

 the first ; these in their turn are marked, and become divided 

 in like manner. This progressive division and subdivision 

 continues in all directions : first downwards, and afterwards 

 horizontally, according to the distance from the surface, which 

 is suitable to the nature of the plant, and always in proportion 

 to the extent of the head which it is called upon to supply. 



These fibres, whether they form the advancing points of the 

 roots and their branches, or as they issue from the exterior of 

 the parts previously formed, are, as has been already noticed, 

 exceedingly sensitive; suddenly extending themselves into 

 water, or into the humid cavities of the soil ; and on a change 

 of these exciting circumstances, as suddenly shrinking back to 

 the place whence they issued, or if exposed to dry air wither- 

 ing away. That this excitement is caused by an impulse of 

 the vital principle within, as well as favoured by the propitious 

 elements without, is sufficiently obvious ; because they are only 

 active simultaneously with the other growing effijrts or expan- 

 sions of the plant, and remain inactive when these are at rest. 

 If, however, they continue to advance, the bases, or first formed 

 parts of the leading ones, gradually change, by enlargement 

 of their diameters, and instead of delicate transparent pro- 

 cesses, become hardened tenacious fibres, thus extending the 

 points and ramifications of the roots on all sides. 



When a root has become as thick as a finger, a transverse 

 section thereof shows, that it is composed of four very distinct 

 parts. First, a central pith, consisting of a spongy, soft, and 

 light substance. Second, a circular body of tough, woody 

 threads ; each forming in the first stage of its existence, a 

 longitudinal sap-conducting tube, either individually, or after- 



