ON STEMS. 45 



appearance of being continuous ; but were it really so, 

 their distance from each other would increase in propor- 

 tion as they diverged from the centre ; yet you see in the 

 carrot they are as close, and consequently much more nu- 

 merous, in the external layers of wood, than in those 

 nearer the central part. In one sense, indeed they may 

 be considered as continuous ; as it is conjectured that the 

 growth of the new wood originates from the extremities 

 of the medullary fibres of the preceding year : this would 

 tend to give regularity to the distribution and direction 

 of the successive rays, and an appearance of continuity. 

 A succession of these horizontal rays, perfectly regular, 

 form vertical planes along the stem, which may be toler- 

 ably well represented by those circular brushes which 

 are made to clean the inside of bottles. 



Emily. The wood of exogenous plants, growing ex- 

 ternally, has not the same difficulties to encounter as 

 that of endogenous plants. 



Mrs. B. The difficulty is rather reversed than dimin- 

 ished, the pressure being from the external upon the in- 

 ternal parts. The first layer surrounding the central pith 

 grows freely during a twelvemonth, but the following 

 year it is enclosed by a new layer ; and notwithstanding 

 the accession of nourishment it receives from the roots, 

 and the additional space it would, if unconfined, occupy, 

 it is pressed and squeezed by the new layer into a nar- 

 rower compass than it occupied the preceding year. In 

 this distressing situation, what is to be done ? Compelled 

 to yield laterally, it makes its way where there is no 

 pressure ; that is to say, vertically : thus the stem grows 

 in height at the same time that it increases in thickness. 

 The first layer of wood having, therefore, found a vent 

 for that new portion of its substance which could not be 

 contained in the contracted space in which it was con- 

 fined by the growth of the second layer, this portion grows 

 freely during the second year ; when a third layer shoot- 

 ing up around and compressing the second, this in its 



214. In what sense may they be considered continuous'? 215. By 

 what may a succession of these horizontal rays be represented'? 216. 

 What does Mrs. B. say of the comparative difficulty of growth in the 

 exogenous and endogenous plants! 217. How does she describe the 

 growth of the former 1 ? 218. When the first layer is greatly com- 

 pressed by the growth of the second, how does it make its escapel 

 219. What takes place the third year! 



