50 



this difiSculty is increased by the fact that the last cells of one year's gro^vth differ from the first 

 cells of the next year's ring only in lorni and not in the thickness of their walls, and therefore 

 produce the same color eiiect. Such cases frequently occur in the wood of the upper half of the 

 disks from limbs (the limb sii]>i)()rted horizontally and in its natural ])ositi()n) and ol'ten the magnifier 

 has to be reinforced by the microscope to furnish the desired information. Fortius (lurpose the 

 wood is treated as in all microscopic work, being lirst soaked in water and then sectioned with a 

 sharp knife or razor and examined on the usual slide in water or glycerin. 



The reason for beginning the counting of rings at the periphery is the same which suggested 

 the marking of all peripheral pieces by the letter a. It is convenient, almost essential, to have, for 

 instance, the thiity-fifth ring in Section II represent the same year's growth as the thirty-fifth ring 

 in Section X. The width of the sapwood, the number of annual rings comixising it, as well as the 

 clearness and uniformity of the line separating the sa])\vood from the lieartwood, are carefully 

 recorded. In the columns of "lemarks" any peculiarities which distinguish the i)articular ])iece of 

 wood, such as defects of any kind, the presence of knots, abundance of resin, nature of the grain, 

 etc., .are set down. 



When finished, a variable number, commonly o to (J small pieces, fairly representing the wood 

 ot the tree, are split off, marked with the numbers of their respective disks, and set aside for the 

 microscopic study, which is to tell us of the cell itself, the very element of structure, and of its 

 share in all the proi^erties of wood. 



The small pieces are soaked in water, cut with a sharp knife or razor, and examined in water, 

 glycerin, or chloriodide of zinc. The relative amount of the thick-walled, dark-colored bands of 

 summer wood, the resin ducts, the dimensions of the common tiacheids and their walls, l)oth in 

 spring and summer wood, the medullaiy rays, their distril)ution and tiieir elements, are the prin- 

 cipal subjects in dealing with coniferous woods; the ((uantitative distribution of tissues, or how 

 much space is occupied by the thick-walled bast, how much by vessels, how nnich by thin-walled, 

 pitted tracheids and i)arenchyma, and how much by the medullary rays; what is the relative value 

 of each as a strength-giving element; what is the space occupied by the lumina, what by the cell 

 walls in each of these tissues — these are among the imi)ortant points in the stuily of the oaks. 



Continued sections from center to jieriphery, magnified '2'i diameters, are employed in finding 

 the relative amount of the summer wood; the limits of the entire ring and that of spring and sum- 

 mer wood are marked on paper with the aid of the camera, and tluas a panoranni of the entire 

 section is brought before the eye. The histology of the wood, the resin ducts, the tracheids and 

 medullary rays, their form and dimensions, are stuilied in tlnn sections "magnified 580 diameters 

 and men more. Any peculiarity in form or arrangement is drawn with the camera and thus graph- 

 ically recorded; the dimensions are measured in the manner described for the measurement of the 

 sununer wood, or with the ocular micrometer. In measuring cell walls, the entire distance between 

 two neighboring lumina is taken as a " double wall," the thickness of the wall of either of the two 

 cells being one-half of this. The advantage of this way of measuring is apparent, since the two 

 points to be marked are in all cases perfectly clear and no arbitrary positions involved. The length 

 of the cells is found in the usual way by separating the elements with Schidtze's solution (nitric 

 acid, chlorate of potassium). All results tabulated are averages of not less than ten, often of more 

 than one hundred measurements. 



In the attempt to find the quantitative relations of the different tissues, as well as the density 

 of each tissue, various ways have been followed. In some cases drawings of magnified sections 

 were made on good, even paper, the different parts cut out, and the paper weighed. In other cases 

 numerous measurements and computations were resorted to. Though none of the results of these 

 attempts can be regarded as perfectly reliable, they have done much to point out the relative im- 

 portance of different constituents of the wood structure, and also the iiossibility and practicability, 

 and even the necessity, of this line of investigation. 



