THE WOOD. 25 



of reserve, materials. Commuiiicatioii between tlii\se aiid the traclieids is effected by means of 

 simi)le pits on their radial walls. 



The inner hark, or phloem, ^>/i (IM. VII, 1 and .V), closely resembles tlie young wood on 

 cross section, its elenuMits being arrangiul m radiating rows and ti:iversed in like manner i)y the 

 medullary rays. The cells composing it ditler, however, in various important jiarticulars from 

 those of the wood. Tiieir walls are of cellulose, and although important as conducting tissue, they 

 contribute comparatively little to the rigidity of the stem. 



lietweeu the wood and inner bark is the cambium or formative tissue, represented in IM. 

 VTl, i, as a light band of extremely small and delicate cells, and iu the same plate 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 ou 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 between the cambium and the tissues derived from 

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

 process, as regards the i)osition of the cell walls, are represented iu I 'I. VIII, 4, iu which the 

 lightest lines represent the youngest walls aiul the heavier ones those of greater age, successively. 

 It is by the constant rei)etition 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 yearly inciease 

 in 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 tbrmed. Tliis results in the strong contrast 

 between the wood last ])roduced iu 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 dearly brought out in 

 PI. VIII, i. 



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 I'ine 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 in them 

 without introducing aTi 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 .'53. 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 Scotiih Pine, for cortex 9 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 nund)er 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 Longleaf Pine, for exam))le, 

 it is seen that while the thick-walled tracheids of the tbrmer make hardly more than the 

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

 Moreover, the gradual, almost imperceptible, transition from spring 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 chietly due. The medullary rays of the different groups of i)ines show certain 

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

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



In all pines the medullary ray is made up of two kinds of colls which diU'er in their general form, .ind still 

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



' Etta I.. KiiDwles, in Botanical Gazette, August, 1886. 



