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; 

 Ktister; Mayr; Penhallow; Raatz). 



TRUE TYLOSES IN CONIFERS 



Tyloses in normal coniferous wood arise chiefly from the parenchyma- 

 tous cells of the medullary rays. (PL 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). 8 



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. 



1 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 ia Abies pectinate, Pinus sylvestris, Pinus 

 strobus, Pinus excel sa, Larix euro pea, and Thuja ccideniali*. 



