xxxii INTRODUCTION. 



and more rarely {e.g. in some MeHacea) the lower part of a compound leaf attains its 

 ftdl size before the yonng leaves or leaflets of the extremity are yet tormed. 



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 onterinost preceding layer and the 

 inner surface of the bark, and by the formation of a new layer or nng of bark withm 

 the innermost preceding layer and outside the new ring of wood, thus forming a succes- 

 sion of concentric cii-cles. The sap elaborated by the leaves finds its way, ma 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 ia layers, the inner ones constitutmg the 

 new ring of wood, and the outer ones the new bark or Uber. 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 theage 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 aifords therefore 

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



213. Mowers 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 tliis 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 afiinity, cannot nevertheless be exclusively relied on in esti- 

 mating the real value of peculiarities of structure. 



214. The flower is considered as a bud (fiomer-had, alaiastrv/m) until the perianth 

 expands, the period of flowering (anthesis) is that which elapses from the first expand- 

 ing of the perianth, till the pistil is set or begins to enlarge, or, when it does not set, 

 until the stamens and pistil wither or fall. After that, the enlarged ovary takes the 

 name cSyoimg fruit. 



215. At the close of the season of growth, at the same time as the leaf-buds or seeds 

 are formed containing the germ of future branches or plants, many plants form also, at 

 or near the bud or seed, large deposits, chiefly, of starch. In many cases, — such as the 

 tubers of a potato or other root-stock, the scales or thickened base of a bulb, the albu- 

 men or the thick cotyledons of a seed, — this deposit appears to be a store of nutriment, 

 which is partially absorbed by the young branch or plant during its first stage of 

 growth, before the roots are sufficiently developed to supply it from without. In some 

 cases, however, such as the fleshy thickening of some stems or peduncles, the pericarps of 

 fruits which perish long before germination (the first growth of the seed), neither the 

 use nor the cause of these deposits has as yet been clearly explained. 



§ 4. Functions of the Organs. 



216. The functions of the Boot ai-e,— 1. To fix the plant in or to the soU or other 

 substance on which it grows, 2. To absorb nourishment from the soil, water, or air, 

 into which the fibres have penetrated (or from other plants in the case of parasites) 

 and to transmit It rapidly to the stem. The absorption takes places through the young 

 growmg extremities of the fibres, and through a peculiar kind of hairs or absorbing 

 organs which are formed at or near those growing extremities. The transmission to 

 the stem is through the tissues of the root itself. The nutriment absorbed consists 



