134 GENERAL CONSIDERATIONS. [BOOKII. 



receives, and returns the superfluous portion 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 plumule 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 simultaneously 

 both in a horizontal and perpendicular direction. This pro- 

 cess may not inaptly be compared to that of weaving, the 

 warp being the perpendicular, and the weft the horizontal, 

 formation. In order to enable the leaf to perform its func- 

 tions of aeration completely, it is traversed by veins originating 

 in the medullary sheath, and has delicate pores (stomates), 

 which communicate 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 cellular 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, connected 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 



