458 
Journal of Agricultural Research 
Vol. I, No. 6 
TYLOSES IN SOFTWOODS 
Coniferous or softwoods lack the large open pores or vessels which 
characterize the hardwoods. They also either lack or show a scanty 
development of wood parenchyma, the chief source of tylose formation 
in the hardwoods. Since it is in relation to the closing of the vessels that 
tyloses are of practical significance, the study of tylose distribution in 
the conifers is of relatively small importance. However, since tyloses 
or tyloselike cells are often present in the tracheids or in the resin canals 
of certain normal coniferous woods, and since they have been found to 
play some part in penetration of wood preservatives and in resin flow, 
their occurrence in the softwoods was studied. 
The occurrence of tyloses in coniferous woods has not received the 
attention given to their occurrence in hardwoods. Often their presence 
has been ignored, or they have been reported as entirely lacking. 1 When 
studied, moreover, investigations were usually confined to parts of the 
plant other than the wood, 2 though there are a few notable observations 
on their occurrence in the wood itself (Boehm; Chrysler; Conwentz; 
Krister; Mayr; Penhallow; Raatz). 
TRUE TYLOSES IN CONIFERS 
Tyloses in normal coniferous wood arise chiefly from the parenchyma¬ 
tous cells of the medullary rays. (PI. LVI, figs, i and 2.) As in the 
hardwoods, it is by the growth of the membranes of the one-sided bor¬ 
dered pits that tyloses are formed, especially where the pits are of large 
size, as in the white pines. In this case tyloses grow into the lumen of 
the tracheid, just as in hardwoods they grow into the vessels or pores. 
Tracheids, like vessels, function as sap conductors, but instead of having 
in their end walls actual openings of considerable size they have only rela¬ 
tively thin regions or pits. These are more or less completely closed by 
an irregularly thickened membrane, portions of which sometimes contain 
very minute perforations (Bailey). Thus in these elements already 
closed or nearly closed, tyloses have not the effect that they have in the 
open vessels of the hardwoods. Moreover, tylose formation of this type 
in conifers can only take place in a comparatively small percentage of the 
tracheids—that is, in those adjacent to the medullary-ray parenchyma 
cells produced as a result of wounds (Boehm; Raatz). 3 
TYLOSELIKE CELLS IN THE RESIN CANALS 
Aside from true tyloses, there is often observed in certain species of 
conifers a partial or complete closing of the resin canals, produced by 
parenchyma cells, but not by growth of the membrane of the one-sided 
1 Reported by Molisch after examining 700 species of plants of all sorts. 
* They are said to be more abundant in the root than in the stem (Raatz). They also have been studied 
in the leaf and in the cone axis. 
8 Boehm and Raatz observed tyloses as a result of wounding in Abies pectinata, Pinus syhestris, Pinus 
strobus, Pinus excelsa, Larix europea , and Thuja occidentals. 
