122 



HISTORY OF THE GROWTH OF A TREE. 



tion of it downwards through the bark to 

 the root ; tubular tissue at the same time 

 appears between the medullary sheath and 

 the bark, thus forming the first ligneous 

 stratum, a part of which is incorporated 

 with the bark, the remainder forming wood. 



During these operations, while the plu- 

 mule is ascending, its leaf forming and 

 acting, and the woody matter created by it 

 descending, the cellular tissue of the stem 

 is forming, and expanding horizontally, to 

 make room for the new matter forced into 

 it ; so that development is going on simul- 

 taneously both in a horizontal and perpen- 

 dicular direction. This process may not 

 inaptly be compared to that of weaving, 

 the warp being the perpendicular, and the 

 Aveft the horizontal, formation. In order to 

 enable the leaf to perform its functions of 

 aeration completely, it is traversed by veins 

 originating in the medullary sheath, and. 

 has delicate pores (stomates,) which com- 

 municate with a highly complex pneumatic 

 system extending to almost every part of 

 the plant. 



Simultaneously with the appearance of 

 woody matter, the emission of young roots, 

 and their increase by addition to the cel- 

 lular substance of their points, take place. 

 They thus are made to bear something 

 like a definite proportion to the leaves they 

 have to support, and with which they must 

 of necessity be in direct communication. 



After the production of its first leaf by 

 the plumule, others successively appear in 

 a spiral direction around the axis at its 

 growing point, all constructed alike, con- 

 nected with the stem or axis in the same 

 manner, and performing precisely the same 

 functions as have been just described. At 

 last the axis ceases to lengthen; the old 

 leaves gradually fall off'; the new leaves, 

 instead of expanding after their formation, 

 retain their rudimentary condition, harden, 

 and fold over one another, so as to be a 

 protection to the delicate point of growth ; 

 or, in other words, become the scales of a 

 bud. We have now a shoot with a woody 

 axis, and a distinct pith and bark ; and of 

 a more or less conical figure. At the axil 

 of every leaf a new growing point had been 

 generated during the growth of the axis ; 

 so that the shoot, when deprived of its 

 leaves, is covered from end to end with 



little, symmetrically arranged, projecting 

 bodies, which are the buds. 



The cause of the figure of the perfect 

 shoot being conical is, that, as the wood 

 originates in 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 exa- 

 mined, it will be found that the interior is 

 no longer imperfectly divided into two por- 

 tions, namely, pith and skin, as it was 

 when first examined in the same way, but 

 that it has distinctly two internal, perfect 

 concentric lines, the outer indicating a 

 separation of the bark from the wood : and 

 the inner, a separation of the wood from 

 the pilh: the latter, too, which in the first 

 observation was fleshy, and saturated with 

 humidity, is become distinctly cellular, and 

 altogether or nearly dry. 



III. "With the spring of the second year, 

 and the return of warm weather, vege'.a- 

 tion recommences. 



The uppermost, and perhaps some other, 

 buds, which were formed the 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 fibrils 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 ; a horizontal 



