XXX OUTLINES OF 



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 attain- 

 ing 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 

 they 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. 



210. The stem, including its branches and appendages (leaves, floral organs, etc.), 

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

 tremity 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 follow- 

 ing season. At a very esrly stage of the development of these buds or seeds, a com- 

 mencement 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 which 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 Meliacea) 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 addition every 

 season of a new layer or ring of wood between the outermost preceding 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 succes- 

 sion 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 con- 

 stituting 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 conse- 

 quently either only becomes more dense without increasing in thickness, or only in- 

 creases by gradual distention, which is never very considerable. It affords therefore 

 no certain criterion for judging of the age of the tree. 



213. Flowers have generally all their parts formed, or indicated by protuberances or 

 growing cells at a very early stage of the bud. These parts are then usually more re- 

 gularly placed than in the fully developed flower. Parts which afterwards unite are 

 then distinct, many are present in this rudimentary state which are never further de- 

 veloped, and parts which are afterwards very unequal or dissimilar are perfectly alike 

 at this early period. On this account flowers in this very early stage are supposed by 

 some modern botanists to be more normal, that is, more in conformity to a supposed 

 type ; and the study of the early formation and growth of the floral organs, called 

 Organogenesis, has been considered essential for the correct appreciation of the affinities 

 of plants. In some cases, however, it would appear that modifications of development, 

 not to be detected in the very young bud, are yet of great importance in the distinction 

 of large groups of plants, and that Organogenesis, although it may often assist in clear- 

 ing up a doubtful point of affinity, cannot nevertheless be exclusively relied on in es- 

 timating the real value of peculiarities of structure. 



214. The flower is considered as a bud {flower-bud, alabastrum) until the perianth 



