82 



MINUTE STRUCTURE OF CONIFEROUS WOOD. 



4, The whole is surrounded by and enclosed in an epidermis or 

 covering familiarly known as the bark. 



To comprehend the histology or minute structure of the parts thus 

 exposed to view, a preparation of the material by certain simple 

 chemical reagents and the aid of the microscope are necessary. For 

 those who desire to study the subject practically, text books devoted 

 to the subject should be consulted ; * a brief sketch of the most 

 important anatomical structures revealed by microscopic examination can 

 only be here given. 



If a three years' old stem of a Scots Pine be substituted for that 

 represented in Fig. 41 it will be found that the rings are more sharply 

 separated from each other; the inner part of the annual 

 zone, or part nearest the centre, is lighter in colour 

 and looser in texture, whilst the outer portion is darker 

 in colour and more compact in texture ; each ring, in 

 fact, shows two zones, the inner representing the spring, 

 the outer the summer growth of each year; the cause 

 of this difference will be presently adverted to. These 

 zones of spring and summer growth are observable in 

 all coniferous wood ; in some species, as in the 

 common Yew, they pass more or less gradually from 

 one into the other; in others, as in Tsuga canadensis, 

 Abies pectinata, Pinus excelsa, they are more sharply 

 defined. The relative dimensions of the spring and 

 summer wood, the width of the annual rings, their 

 uniformity or want of uniformity, have considerable 

 influence on the properties and value of the timber. 



The pith in the centre of the stem is composed of 

 cells with cellulose walls f which when first formed are 

 filled with protoplasm which disappears as the formation 

 of the cambium layers and woody tissues derived from 

 them proceeds. Under a high magnifying power the 

 cambium or formative tissue is seen to consist of cells 

 in radial rows arranged with considerable regularity. The 

 cells are filled with protoplasm in which a nucleus can 

 often be detected; the growth of the stem and branches 

 proceeds by the division of these cells by longitudinal 



with bordered pits as b. walls. 



Etrach^id^withrmed 1 "! 8 - The ligneous element of the stem consists of fibre 



lary ray. x 250. technically called proseiichymatous tissue. This tissue is 



composed of elongated fusiform cells enclosing a narrow 



cavity and whose ends are dove-tailed between one another. The 



individual cells are termed tracheides; they are formed from the 



cambium by cell division and have ligneous walls which show on 



their inner side lines of striation and certain irregularities of growth 



that have obtained the name of "bordered pits." As the formation 



of new tracheides proceeds, the walls of the older ones become 



gradually thickened till the original cavity is quite filled up ; they then 



* Such as "Practical Botany" by Bower and Vines; Scott's " Introduction to Structural 



Botany," etc. 



f Cellulose is secreted from the protoplasm ; it is the primitive membrane of the cell 

 free from all matter subsequently taken up by the roots and deposited within it. The 

 cell is the common starting point of all elementary organs, and protoplasm is the formative 

 and living part of the cell. 



