ROLE OF THE MICROSCOPE 181 



Rays 



The height, width, and structure of rays have been commonly used 

 by most investigators, who have endeavored to classify the woods of 

 Coni ferae and Dicotyledons. The variations, that are liable to occur in 

 these anatomical characters, may be illustrated by two examples ; one 

 taken from the Coniferse and one from the Dicotyledons. 



Figures 16 and 17 are photomicrographs of tangential longitudinal 

 sections of two specimens of redwood. Sequoia sempervirens Endl. 

 In the former, the rays are one cell wide, uniseriate, a type that is 

 characteristic of most conifers that do not possess horizontal resin 

 passages. In the latter, they are biseriate and triseriate, types that 

 occur in many Dicotyledons. 



As is well known, the wood of the chestnut is commonly distin- 

 guished anatomically from that of oak by the structure of the rays. In 

 chestnut, the rays are uniseriate; in oak, wide, miiltiseriate rays are 

 present in addition to the narrow^ sheets of storage tissue. However, 

 it has been found that this distinction breaks down when applied to 

 the oaks and chestnuts of tropical and subtropical regions. Even in 

 our northern species of these genera, the distinction cannot be ap- 

 plied to tissue grown under all environmental conditions. Figure 18 

 illustrates a cross section of the rootwood of white oak (Quercus alba 

 L.). In this specimen the rays are entirely uniseriate. Figure 19 shows 

 the presence of wide rays in the stem wood of Castanea dentata 

 (Marsh) Borkh., and illustrates the type of tissue that may be 

 formed by the chestnut when it is attacked by Bndothia parasitica Mur. 



Pits and Tertiary Spiral Thickenings 



It will be well, perhaps, to consider the variability of some of 

 the more minute anatomical characters that are only visible under 

 comparatively high magnifications. The form, structure, and distri- 

 bution of the minute pits which occur in the walls of tracheary and 

 parenchymatous elements have frequently been used as diagnostic 

 criteria.^ For example, in distinguishing the wood of Taxodium from 

 that of Sequoia, use has been made of the pits between the lateral 

 walls of the ray cells and the adjacent walls of the tracheids. In 

 Taxodium, the elongated orifices of the pits are said to be diagonal to 

 the long axis of the ray cells. In Sequoia, on the contrary, they are 



5 Penhallow, D. P. A manual of the North American Gymnosperms, page 

 196. Ginn & Co., Boston, 1907. 



