22 TIMBER AND TIMBER TREES. 



of the walls of these tubes, or elements, the result being 

 that a piece of wood turns out to have a complexity of 

 structure far beyond anything that was supposed to 

 exist by the older observers. 



But structure suggests function. When we see a 

 piece of machinery, we are not satisfied with knowing 

 how its parts are put together, and what these parts are 

 composed of ; we, almost instinctively, ask, how do the 

 parts work, and what duties do they perform, severally 

 and collectively. So .with the botanist — not content 

 with knowing the structure and origin of the parts, or 

 tubes, themselves, or with observing how they are 

 grouped or arranged, he at once proceeds to inquire 

 what they do. 



Putting the results of such investigations quite 

 generally, it is found that all wood, whether in small 

 quantity as in herbaceous plants, or in large quantity 

 as in timber, may be broken up into long or short, open 

 or closed, wide or narrow, thick or thin-walled tubular 

 elements. 



Some of these are not much longer than broad, and 

 may be compared to boxes of parallelopiped or shortly 

 prismatic shape, and are termed, generally, cells. The 

 individual differences between the cells of one wood and 

 another, or the wood from one part of a plant and that 

 from another, chiefly, depend on their number and states 

 of aggregation— in layers, clumps, long tracts, etc. — 

 on the nature of their contents, including colour— on the 

 thickness of their walls, as well as the chemical sub- 

 stances (including pigments) to be extracted therefrom 

 —and on the form and size, etc., of the markings on 

 these walls, due principally to irregular or regular 

 difference of thickness. 



Others of these elements— to pass to the extreme 



