INCREASE OF BOOTS IN DIAMETER. 21 



cleft; conditions, the real nature of which is unknown, cause the muoUage 

 to press against the cleft; the mucilage is iatroduced, it organises, solidi- 

 fies, and the point of a root is established ia the cleft..' The point forms 

 more mucilage ia advance ; that also solidifies, and a further lodgment 

 is made ; and thus the growth goes on, through all the sinuosities of the 

 narrow passage that it traverses. In the annexed figure, the space from 

 o to 6 represents the part where the root passed through the pipe, which 

 must have been nearly two and a half inches thick ; the root, there, 

 was as tldn as paper, and had followed every bend in the crack. As 

 soon as it reached the inside of the pipe ^at b) it swelled, acquired its 

 usual cylindrical form, and thence proceeded to develop and branch in 

 the manner already described. The thiu connecting plate was sufficient 

 to maintain the vitality of the roots for many years. 



The only known exceptions to the rule that roots do not 

 lengthen by a general distension of their tissue, occur in parts 

 growing in air or water, which are non-resisting media, or in 

 certain 'endogenous trees, whose roots lengthen to such a 

 degree as to hoist the trunk up into the air, off the ground with 

 which it at first was level. 



It is not, however, merely in length that the root increases ; 

 if such were the case, all roots would be mere threads. They 

 also augment in diameter, simultaneously with the stem, and 

 under the influence of exactly the same causes. Neither is it 

 by an embryo alone that roots are formed. A plant, once in a 

 state of growth, has the power' of produciug roots from various 

 parts, especially from leaves and stems. 



The well-known emission of roots by the stems of the common Laurel 

 is a phenomenon due, as it seems, to the death of the lower part of the 

 stem, the live part of which is thus compelled to organise its descending 

 sap in the form of roots. Vines in damp hot-houses, with their roots 

 in a cold border, habitually exhibit the same tendency. And as a further 

 illustration, one published by Mr. W. B. Booth may be introduced. 

 This was the case of a Spanish Chestnut between ninety and one hundred 

 years old, and of considerable size,, cut down in 1849. With the excep- 

 tion of its foliage, which always had a yellowish, sickly tinge, there 

 was scarcely anything else about it that indicated decay. Its trunk 

 seemed perfectly sound, and the young shoots it annually made, 

 appeared to be pretty strong and healthy. No sooner, however, had 

 the workmen commenced cutting, than it was discovered that for ten 

 feet high, as much as two-thirds of the bark round the trunk was dead 

 and reduced to a mere shell. On removing this thin covering, the sap- 



