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bark or liber. In most exogenous trees, in temperate climates, the seasons of 

 growtL 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 distension, 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 j)arts formed, or indicated by protuberances 

 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 which afterwards unite 

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

 ther developed, and parts which are afterwards very unequal or dissimilar are per- 

 fectly 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 confor- 

 mity to a supposed type; and the study of the early formation and grow^th of the 

 floral organs, called Organogenesis^ has been considered essential for the correct ap- 

 preciation 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 clearing up a doubtful point of affinity, cannot 

 nevertheless be exclusively relied on in estimating the real value of peculiarities of 

 structure. . 



214. The flower is considered as a hud (flower-lud, alalasirum) until the perianth 

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

 panding 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 of youn/z/rm'^ 



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 albumen 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 germind- 

 Hon (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 Root are,— 1. To fix the plant In or to the soil, 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 i)lants in the case of para- 

 sites), and to transmit it rapidly to the stem. The absorption takes place through 

 the young growing 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 chiefly of carbonic acid and nitrogen or nitrogenous compounds ; 

 dissolved in water. 3. In some cases roots secrete or exude small quantities of 

 matter in a manner and with a purpose not satisfactorily ascertained. 



217. The Stem and its branches supnort the leaves, flowers, and fruit, transmit the ^ 

 crude sap, or nutriment absorbed by the roots and mixed with previously organized 

 matter, to the leaves, and retransmit the assimilated or elaborated sap from the leaves 

 to the growing parts of the plant, to be there used up, or to form deposits for future 

 use (204). The transmission of the ascending crude sap appears to take place chiefly 

 through the elongated cells associated with the vascular tissues^ passing ^om one cell 

 to another by a process but little understood, but known by the name of endosmos^' 



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