i 



XXX , IXTUODTJCTION. 



cells (100) are formed. These cells separate in layers, the inner ones constituting tlie 

 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 sufliciently distinct to indicate the age of the tree ; but in many tropical 

 and some eyergreen 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 withoiit increasing in thickness, or only m- 

 creases by gradual distention, whicli is never rcry 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 rc- 

 giJarly 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 after^'ards very unequal or dissimilar arc 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 

 Orga nor/en esis,hix^ been considered essential forthccorrect appreciation of tlie afTinitieS 

 of plants. In some cases, however, it would appear tliat 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 atructui^e. 



214. The flower is considered as a hud {J!awer-bud, alabasinrm) until the perianth 

 expands, ihe period of fiowering {antliesis) 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 oi young fruit, 



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

 arc 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,— tliis deposit appears to be a store of nutriment, 

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

 growth, before the roots are sufBciently developed to supply it from witliout. 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 Eoot are,— 1. To fix the plant in or to the soil or other 

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

 mto winch the fibres liave penetrated (or from other plants in the case of pamsites), 

 and tx) transmit it rapidly to the stem. The absorption takes place through tlie 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 13 through the tissues of the root itself. The nutriment absorbed consists ; 

 chiefly of carbonic acid and nitrogen or nitrogenous compounds dissolved in Avater- 

 3. In some oases roots secrete or exude small quantities of matter in a manner and 

 witli a purpose not satisfactorily ascertained. 



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

 crude sap, or nutriment absorbed by the roots and mixed with previously organize*^ 

 matter, to the leaves, and re-tninsmit tlio assimilated or elaborated sap from thclcatea 

 to the groAA-ing parts of the plant, to be there used up, or to form deposits for future 



(201). The transmission of the ascending crude sap appears to take place chiefl.V 

 through the elongated cells associated with the vascular tissues, passing from one cell 

 to another by a process but little understood^ but known by the name oiendomose. 



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