238 PHYSIOLOGY. BOOK II. 



shoot bein^ conical is, that, as the wood originates from the 

 base of the leaves, the lower end of the shoot, which has the 

 greatest number of strata, because it has the greatest number 

 of leaves above it, will be the thickest ; and the upper end, 

 which has had the fewest leaves to distend it by their deposit, 

 will have the least diameter. Thus that part of the stem 

 which has two leaves above it will have wood formed by two 

 successive deposits ; that which has nine leaves above it will 

 have wood formed by nine successive deposits ; and so on : 

 while the growing point, as it can have no deposit of matter 

 from above, will have no wood, the extremity being merely 

 covered by the rudiments of leaves hereafter to be developed. 



If at this time a cross section be examined, it will be found 

 that the interior is no longer imperfectly divided into two 

 portions, namely, pith and skin, as it was when first ex- 

 amined in the same way, but that it has distinctly two, internal, 

 pei'fect, concentric lines, the outer indicating a separation of 

 the bark from wood ; and the inner, a separation of the wood 

 from the pith : the latter, too, which in the first observation 

 was fleshy, and saturated with humidity, is become distinctly 

 cellular, and altogether or nearly dry. 



With the spring of the second year and the return of 

 warm weather vegetation recommences. 



The uppermost, and perhaps some other, buds which were 

 formed tlie previous year gradually unfold, and pump up sap 

 from the stock remaining in store about them ; the place of the 

 sap so removed is instantly supplied by that which is next it ; 

 an impulse is thus given to the fluids from the summit to the 

 roots ; fresh extension and fresh fibrOs are given to the 

 roots ; new sap is absorbed from the earth, and sent upwards 

 through the wood of last year ; and the phenomenon called the 

 flow of the sap is fully completed, to continue with greater or 

 less velocity till the return of winter. The growing point 

 lengthens upwards, forming leaves and buds in the same way 

 as the parent shoot: in like manner also each bud sends down 

 its roots, in the form of fibres within the bark and above the 

 wood of the shoot from which it sprang ; thus forming on the 

 one hand a new layer of wood, and on the other a fresh 

 deposit of bark. In order to facilitate this last operation, the 



