THE WOOD. 25 



of reserve materials. Communicatiou betweeu these and tlie tracbeids is effected by means of 

 simple pits on their radial walls. 



The inner bark, or phloem, ph (PI. VII, 1 and .5), closely lesembles the young wood on 

 cross section, its elements being arranged in radiating rows and traversed in like manner by the 

 medullary rays. The cells composing it diH'er, however, in various important particulars from 

 those of the wood. Their walls are of cellulose, and although iinpnrtant as conducting tissue, they 

 contribute comjiaratively little to the rigidity of the stem. 



Between the wood and inner bark is the cambium or formative tissue, re])resented in PL 

 VTl, 1, as a light band of extremely small and delicate cells, and in the same idate as a zone 

 of cells with thin walls and large lumen, contrasting strongly with the wood elements and those of 

 the inner bark between which they lie. It is from the cells of the cambium that those of the wood 

 are formed on the one hand and those of the bark on the other. The process is a gradual one, and 

 no absolute line of demarcation can be drawn betweeu the cambium and the tissues derived from 

 it. The cells of the cambium multiply by tangential division. The essential features of this 

 process, as regards the position of the cell walls, are represented in PI. VIII, ■/, in which the 

 lightest lines represent the youngest walls and the heavier onei those of greater age, successively. 

 It is by the constant repetition of this process of tangential division and the subsequent Thicken- 

 ing of the walls of the cells thus formed that the wood and inner bark make their j-early increase 

 iu thickness. In the spring the cells of the cambium are large and vigorous, and a rapid forma- 

 tion of wood elements with relatively thin walls and large cavities takes place, while later in the 

 season much smaller tracheids with thicker walls are formed. This results iu the strong contrast 

 between the wood last produced in any given year and that formed at the beginning of the next 

 season's growth, giving rise to the sharp distinction of annual rings so clearly brought out iu 

 PI. VIII. :?. 



The histological characters thus briefly summarized hold true, in a general way, for other 

 conifers as well as the "White Pine. This species, however, presents a number of peculiarities 

 that are of both physiological and economical interest. 



The resin ducts of the White Pine are larger and more numerous in the cortex than in the 

 wood, an arrangement well adapted to secure the protective action of the resin contained iu them 

 without introducing an element of weakness into the wood. Comparisons with other species bring 

 out this fact in a striking manner. Thus, upon comparing the distribution of the resin ducts in 

 stems of the White and Scotch pines, as nearly alike as possible, it was found that in the cortex 

 of White Pine stems of one year's growth the number of resin passages ranged from 20 to 47, the 

 average being about 3.3. The number in the wood was more uniform and averaged about 13. In 

 the Scotch Pine the average for the wood was found to be 33 and for the cortex 10. Taking the 

 second year's growth in the same way, the average number for cortex of White Pine in the 

 specimens examined was 28 and for wood 27; in Scotch Pine, for cortex and for wood 37.' 

 The small size of the resin ducts in the wood contrasts strongly with the very large ones of Scotch 

 Pine, which seriously interfere with the continuity of the wood and tend both to weaken it and to 

 give it an uneven texture. 



The extremely small number of thick walled tracheids constituting the summer wood of the 

 White Pine is in marked contrast with the broad band of summer wood formed in various other 

 species. Comparing the annual rings of White Pine with those of Lougleaf Pine, for example, 

 it is seen that while the thick-walled tracheids of the former make hardly more thaii the 

 mere outer edge of each ring, those of the latter constitute one-third or more of its entire width. 

 Moreover, the gradual, almost imperceptible, transition from si)ring to summer wood in the 

 White Pine contrasts strongly with the abrupt line of demarcation seen in Longleaf Pine and all 

 other Yellow Pines. It is to this very gradual transition that the uniform texture of the wood 

 of White Pine is chiefly due. The medullary rays of the different groups of i)ines show certain 

 structural peculiarities that appear to be constant for the group of species in which they occur. 

 The writer is indebted to Mr. Filibert Itoth for the ftdlowing notes in regard to this feature: 



In all iiines the medullary ray is made up of two kinds of cells which ditier iu their general form, and still 

 more in the configuration of the cell wall and pits. The one kind occupies the upper and lower rows of each ray, 



' Ett;\ L. Kuiiwles, in Botanical Gazette. Aunust, 1886. 



