VARIATION IN THE SIZE OF RAY PITS OF CONIFERS.* 
Forest B. H. Brown. 
Since Haeckel proposed the word Ecology in 1886, there has 
been an ever growing interest in the influence which environmental 
factors may have in determining the form and structure of plants. 
“Anatomy, particularly stimulated by Haberlandt, has recently 
been greatly enriched by numerous researches dealing with the 
question of the harmony between structure and environment.” * 1 
Trees of the same species, but grown under different conditions, 
will show differences in the structure of their woody tissues that 
materially affect the durability, strength, and other properties 
of the wood. In a general way, many of such structural differences 
have been related to the conditions under which the tree was 
grown. 
To some extent, at least, the physical factors may influence 
the structure of wood. Cieslar 2 found that certain conifers would 
form “Rotholz,” a tissue of great strength under compression, 
due to the mechanical influence of a one-sided crown or the weight 
of a branch. But since one of the main purposes of the woody 
elements of a tree is to conduct and store the products of assimila¬ 
tion, and to convey the watery solutions, gathered by the roots, 
to the leaves and other parts where they may be needed, it may be 
inferred that factors more directly related to the vital processes 
of the tree will also be more directly related to structural varia¬ 
tions. 
Of the tissues which go to make up the woody part of the stem 
of coniferous trees, the medullary ray is one of the most complex, 
in both its structural and functional aspects. While they make 
up only 4-8 % of the volume of the wood, their height and width 
is so small that often over 2,500 rays may be counted in one sq. 
cm. on the tangential surface (Fig. 1). The average volume 
of a typical coniferous ray shown in this plate is but one twentieth 
that of a fine silk thread. None the less, the ray of Picea and 
Larix, the genera selected for comparison in this paper, is com¬ 
posed of at least two kinds of tissue with an accompanying dif¬ 
ference in function (Fig. 2). At the margins are the ray tracheids 
(r.t.), which communicate with the adjacent wood tracheids by 
means of bordered pits. “Their purpose is to facilitate the transfer 
of water radially between the tracheids.” 3 Distinguished from the 
* Contribution from the Botanical Laboratory of the Ohio State Uni¬ 
versity, No. 90. 
1. Warming. 1909. Ecology of Plants, p. 3. 
2. Centralblatt f. d. gesamte Forstwesen. Apr., 1896. 
3. Strasburger. 1908. Bonn Text-Book, p. 140. 
542 
