THE LEAF— THE COTYLEDONS. 



37 



Some of the cells of this fundamental tissue 

 remain unchanged, others become differentiated 

 or converted into tubes and vessels of different 

 character. The most important change, from 

 our present point of view, is the formation of 

 the woody bundles. These in an ordinary tree 

 or shrub consist of cells, some unchanged or 

 relatively so, others converted into thick-walled 

 tubes or vessels, and 

 these constitute the wood 

 of the bundle turned to- 

 wards the centre of the 

 stem. Other of the con- 

 stituent cells lengthen 

 into tubes with delicate 

 thin walls, and these 

 constitute the " bast " 

 on the outer side of 

 the bundle. Between 

 the bast and the wood 

 are some active growing 

 cells constituting the 

 primary cambium. 



When the second 

 season's growth begins, 

 these cambial cells develop a layer of second- 

 ary wood on the inner, and a layer of secondary 

 bast on the outer side of the first ring, and so 

 it comes about that the wood of most trees and 

 shrubs presents to the eye, when cut across, a 

 series of concentric rings traversed hy radiating 

 medullary rays. As in this country in ordinary 

 seasons one such ring constitutes a season's 

 growth, it follows that the number of rings 

 offers a very close approximation to the number 

 of years that the tree has lived. But allowance 

 has to be made in certain cases for the fact that 



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 f 



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if 1 ' 



rip' 





1 



7 1 



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ill :■' 

 I'Jf • ■■' 



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jljj 



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Mi 



Fig. 41.— Section of Exogenous Stem, 

 showing central pith, three zones 

 of wood, and bark on the outside 

 (diagrammatic). 



Fig. 42.— Section of Endogen and Acrogen. 



a ring may be imperfectly formed in some 

 seasons, or that a second may in other seasons 

 be developed in the same year. The number 

 of the rings is, however, a pretty sure approxi- 

 mate index of the age of the tree or branch; 

 whilst their thickness and condition afford an 

 indication of the climatal vicissitudes to which 

 they were subjected in any particular year. 

 The structures and arrangements above de- 



scribed apply only to ordinary trees and shrubs, 

 such as spring from seedling plants that produce 

 two cotyledons or seed-leaves. The structure 

 of the stem and of its constituent bundles varies 

 greatly in plants the seedlings of which have 

 but one seed-leaf, and in ferns. In palms, for 

 instance, the vascular bundles are not arranged 

 in rings of wedge-like masses, but are dotted 

 irregularly through the fundamental tissue, and 

 the constitution of the bundles is different. 

 Still more varied is the state of things in Tree 

 Ferns, wherein the bundles are arranged in 

 wavy plates in the midst of the fundamental 

 tissue (fig. 42). For full accounts of these diver- 

 sities recourse must be had to the ordinary text- 

 books. Their relation to practical cultivation 

 has not yet been made fully apparent. 



[M. T. M.] 



CHAPTER V. 



THE LEAF— THE COTYLEDONS. 



Forms of the Leaf — Simple Leaves — Compound 

 Leaves — Leaf-structure — Epidermis — Pairs — 

 Leaf-action — Evaporation — Respiration — Assi- 

 milation. 



The presence of a more or less flat, green 

 plate, or of a series of such plates, is an ex- 

 tremely frequent characteristic of plants. We 

 saw it in the simplest form in the "thallus" 

 of sea -weeds, in some of which, moreover, it 

 branches and assumes forms outwardly as com- 

 plicated as those which are met with in the higher 

 plants. In Mosses the stem with its projecting 

 leaves is very prominent, whilst in the Ferns the 

 leaf forms the most prominent part of the whole 

 plant. The leaf is also generally very well 

 marked in the higher seed-bearing plants, and, 

 in alluding to the seedling plant of the pen- 

 stemon, we have spoken of the two first leaves 

 raised upon the caulicle as seed-leaves or "cotyle- 

 dons" (fig. 28). Plants that have two such seed- 

 leaves, as the penstemon, are spoken of as Dicoty- 

 ledonous, others that have one are called Mono- 

 cotyledonous, as the wheat or the lily. The presence 

 of one or of two cotyledons, as the case may 

 be, is associated with other differences, so that 

 when a plant is classed as a "Dicotyledon" we 

 do not mean simply that its embryo or seedling 

 plant produces two cotyledons, but also that its 

 wood is in rings, as previously explained, whilst 

 there are other differences in the leaves and 

 flowers which will be mentioned later on. The 

 classification of all seed -bearing plants into 

 Monocotyledons or Dicotyledons is very im- 



