INTRODUCTION. XXIX 



usually is in commuiiicatio]! with the oraiy bj- a channel running down the centre of 

 tlie style. 



204. Tubers, fleshy thickenings of the stem or other parts of the plant, succulent 

 leaves or branches, the flesliy, woody, or bony parts of fruits, the albumen, and the 

 thick fleshy parts of embryos, consist chiefly of largely developed cellular tissue, re- 

 plete with starch or other substances (192), deposittsd ajjparcntly in most cases for the 

 eventual future use of the plant or its parts wlien recalled into activity at the approach 

 of a new season. 



205. Hairs (171) are usually expansions or processes of the epidermis, and consist 

 of one or more cells placed end to end. AVhen thick or hardened into prickles, they 

 still consist usually of cellular tissue only. Thorns (170) contain more or less of a 

 fibro- vascular system, accordhig to their degree of development. 



-06. Glands, in the primary sense of the word (175, 1), consist usually of a rather 



loose cellular tissue without ejiidermis, and often replete with resinous or other sub- 

 stances, 



§ 3. Growth of the Organs. 



207.^ Roots grow in length constantly and regularly at the extremities only of their 

 fibres, in proportion as they find the requisite nutriment. They form no buds contain- 

 ing tlie germ of future bmnches, but their fibres proceed irregularly from any part of 

 their surface witliout previous indication, and when their growth has been stopped for 

 a tune, either wholly by the close of the season, or partially by a deficiency of nutii- 

 tnent at any particular spot, it will, on the return of favourable circumstances, be re- 

 f-uuicd 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 \\\\ or other- 

 wise injured, or stopped by a rock or other obstacle opposing its progress, lateral iibres 

 will be formed on tlie stiiriiving portion i thus enabling the root as a whole to diverge 

 ^ any direction, and ti-.-ivel for and wide when lured on by appropriate nutriment. 

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

 ing at once then- 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 midorit ; and the new cells, constituting a greater or 

 lesser portion of the ends of the fibres^ remain some time in a gi-owing state before 

 they have attained then' full size. 



209. The roota of Exogens, when perennial, increase in thickness like stems by the 

 addition of concentric layers, but these are usually much less distinctly marked ; and 

 "1 a large number of perennial Exogens and most Endogens the roots are annual, 

 pmshing at the close of the season, fresh adventitious roots springing from the stock 

 when vegetation commences the following season. 



^10,^ The Stem, including its branches and appendages (leaves, floral organs, ete.), 

 grows in length by additions to its - extremity, but a much greater proportion of the 

 extremity and branches remains in a growing and expanding stale lor a much longer 

 yme than in the case of the root. At the close of one season, leaf-buds or seeds are 

 ronned, caeh containing the germ of a branch or young plant to be produced the 

 lollownig season. At a verv early stage of th.e development of these buds or seeds, a 

 commencement may be found of many of the leaves it is to bear; and before a leaf 

 ^^nlolds, every leaflet of whi.h it is to consist, every lobe or tooth which is to mark its 

 JV^^gin, may often be traced in miniature, and thenceforth till it attains its full size, 

 ^ onmch grows and expands in every pnrt. In some cases however the lower part of 

 a bnuich and more rarely {e.g. in some Meliacece) the lower part of a compound Ici 

 tains Its full size before tiie young leaves or leaflets of the extremity are yet formec 

 -=^11. The perennial stem, if exogenous (198), grows in thickness by the addition every 

 ison of a new layer or ring of wood between the outermost preceding layer and the 

 "nner surftice of the bnrk, and by the formation of a new layer or ring of bark witlim 



attai 



d. 



season 



ceUs 



connecting the wood with the bark, by the division and enlargement of which new 

 VOL. I. ' ' d 



