826 SUMMARY OF CURRENT RESEARCHES RELATING TO 



spiral thickening of the wall is not confined to the tracheida, but ex- 

 tends also to the outer cells of the medullary rays of the first-formed 

 annual ring. Here they are especially conspicuous on the horizontal 

 walls, and therefore distinct on transverse section ; and are found also 

 in the later annual rings where there is no trace of spirally thickened 

 tracheids. The spiral thickening was found most distinct in the 

 tracheids of the yew and of Abies Douglasii. 



Comparative Anatomy of the Tissue of the Medullary Rays and 

 Annual Zones of Growth in Conifers.* — Herr H. Fischer has made 

 a careful study of this subject in the stem, root, and branches of Finns 

 Abies. Notwithstanding contrary statements of previous observers, 

 he states that no absolute universal character by which the wood of 

 the stem, root, and branch can be distinguished from one another is to 

 be obtained from the relationship between the mean number and 

 height of the medullary rays in the successive annual rings and the 

 age of the ring. The stem and branch can, however, be distinguished 

 by the different structure of their broad annual rings. In the stem 

 these consist, as a rule, chiefly of summer- wood, in the branch chiefly 

 of autumn-wood. In narrow annual rings in both stem and branch, 

 the autumn-wood constitutes at least half the breadth, in the branch 

 usually more. The stem, root, and branch are to be distinguished by 

 the anatomical structure of their annual rings, according as they are 

 wide or narrow. In narrow rings in the root, the summer-wood 

 usually prevails ; vride rings contain as a rule more summer- wood 

 than rings in the stem of the same width. A prevalence of autumn- 

 wood is sometimes seen in very young and narrow rings of older roots. 

 There is only a gradual passage between the character of the wood in 

 primary and secondary roots. The annual rings are usually well 

 marked in the stem, root, and branches, the chief exceptions being 

 furnished by the root. 



Behaviour of the Leaf-trace-bundles of Evergreen Plants as the 

 stem increases in thickness.t — Dr. 0. Markfeldt has investigated the 

 question, What becomes of the leaf-trace — the common bundles of 

 vascular plants which, within the stem, represent the discernible trace 

 of the leaves to which they belong — when a new annual ring is formed 

 in each recurring vegetative period, in the case of those trees and 

 shrubs which retain their leaves through the winter ? His observa- 

 tions were chiefly directed to Gymnosperms and Dicotyledons, and 

 with the following general results : — 



All the Gymnosperms examined have a portion of the leaf-trace 

 which runs through the cortex, parallel to the main axis of the stem or 

 branch, and surrounded on its lower side by cambium. The portion of 

 the leaf-trace which runs into the wood stands vertically to the axis, 

 or nearly so, and is closely inclosed on its upper and inner sides by 

 the wood. The annual increase in thickness of the stem or branch 

 causes a rupture of the leaf-trace in the neighbourhood of the 

 cambium, while at the same time new vascular elements are formed 



* Flora, Ixviii. (1885) pp. 268-294, 302-9, 313-24 (1 pi,), 

 t Ibid., pp. 33-9, 81-90, 99-113 (1 pi.). 



