OUTLINES OF BOTANY. xliii 



their growth has been stopped for a time, either wholly by the close of the 

 season, or partially by a deficiency of nutriment at any particular spot, it will, 

 on the return of favourable circumstances, be resumed at the same point, if 

 the growing extremities be uninjured. If during the dead season, or at any 

 other time, the growing extremity is cut off, dried up, or otherwise injured, or 

 stopped by a rock or other obstacle opposing its progress, lateral fibres will 

 be formed on the still living portion ; thus enabling the root as a whole to 

 diverge in any direction, and travel far and wide when lured on by appropriate 

 nutriment. 



208. This growth is not however by the successive formation of terminal cells 

 attaining at once their full size. The cells first formed on a fibre commencing 

 or renewing its growth, will often dry up and form a kind of terminal cap, 

 which is pushed on as cells are formed immediately under it ; and the new cells, 

 constituting a greater or lesser portion of the ends of the fibres, remain some 

 time in a growing state before tbey have attained their full size. 



209. The roots of Exogens, when perennial, increase in thickness like stems 

 by the addition of concentric layers, but these are usually much less distinctly 

 marked ; and in a large number of perennial Exogens and most Endogens the 

 roots are annual, perishing at the close of the season, fresh adventitious roots 

 springing from the stock when vegetation commences the following season. 



110. The Stem, including its branches and appendages (leaves, floral organs, 

 etc.), grows in length by additions to its extremity, but a much greater pro- 

 portion of the extremity and branches remains in a growing and expanding 

 state for a much longer time than in the case of the root. At the close of one 

 season, leaf-buds or seeds are formed, each containing the germ of a branch or 

 young plant to be produced the following season. At a very early stage of the 

 development of these buds or seeds, a commencement may be found of many 

 of the leaves it is to bear ; and before a leaf unfolds, every leaflet of which it is 

 to consist, every lobe or tooth w r hich is to mark its margin, may often be traced 

 in miniature, and thenceforth till it attains its full size, the branch grows and. 

 expands in every part. In some cases however the lower part of a branch, and 

 more rarely (e.g. in some Meliacece) the lower part of a compound leaf, attains 

 its full size before the young leaves or leaflets of the extremity are yet formed. 



211. The perennial stem, if exogenous (198), grows in thickness by the addi- 

 tion every season of a new layer or ring of wood between the outermost prece- 

 ding layer and the inner surface of the bark, and by the formation of a new layer 

 or ring of bark within the innermost preceding layer and outside the new ring of 

 wood, thus forming a succession of concentric circles. The sap elaborated by 

 the leaves finds its way, in a manner not as yet absolutely ascertained, into the 

 cambium-region, a zone of tender thin-walled cells connecting the wood with 

 the bark, by the division and enlargement of which new cells (190) are formed. 

 These cells separate in layers, the inner ones constituting the new ring of 

 wood, and the outer ones the new bark or liber. In most exogenous trees, in 

 temperate climates, the seasons of growth correspond with the years, and the 

 rings of wood remain sufficiently distinct to indicate the age of the tree ; but in 

 many tropical and some evergreen trees, two or more rings of wood are formed 

 in one year. 



212. In endogenous perennial stems (199), the new wood or woody fibre is 

 formed towards the centre of the stem, or irregularly mingled with the old. 

 The stem consequently either only becomes more dense without increasing in 

 thickness, or only increases by gradual distention, which is never very consider- 

 able. It affords therefore no certain criterion for judging of the age of the 

 tree. 



213. Elowers have generally all their parts formed, or indicated by protu- 

 berances or growing cells at a very early stage of the bud. These parts are 

 then usually more regularly placed than in the fully developed flower. Parts 



