Mar. 25 , 1914 Tyloses in American Woods 459 



bordered pit. Such growths are termed "tyloselike," since they produce 

 an effect very similar to that produced by the true tyloses of the hard- 

 woods. 



Resin canals or ducts are normally present in the following coniferous 

 genera: Larch, or tamarack (Larix), spruce (Picea), Douglas fir (Pseu- 

 dotsuga), and pine *(Pinus). These canals when seen in cross section 

 often bear a superficial resemblance to the vessels or pores of the hard- 

 woods. (PI. LVII, fig. i.) They are, however, different in both their 

 origin and function. Resin ducts are not cellular elements, but simply 

 intercellular spaces which result from the splitting apart of the common 

 walls of a group of parenchyma cells. A very early stage of this splitting 

 is shown in Plate LVI, figure i. These parenchyma cells which surround 

 the canal opening are called "epithelial cells." They are the seat of 

 resin formation, and they cause the tyloselike closing of the resin canal. 

 Certain of them often remain thin walled and contain plasma. (PI. LVIII, 

 figs. 2 and 5.) After they split apart to form the canal, when they change 

 in shape and size, a further swelling and growth may take place which 

 closes the canal entirely or in part. (Pis. LVII, figs, i and 2, and LVIII, 

 figs. 2, 5, and 6.) The fact that it is the growth or expansion of the 

 whole cell, and not a portion of the wall of that cell, together with a 

 portion of the wall of the neighboring cell, as in the tylose-forming 

 membrane of the one-sided bordered pits of the hardwoods, clearly 

 indicates the difference between the true tyloses of the hardwoods and 

 the tyloselike cells in the resin canals of the conifers. 



OCCURRENCE OF TYLOSES AND TYLOSELIKE CELLS IN NATIVE 



CONIFERS 



Over 600 permanently mounted sections from coniferous woods in 

 the collection of the Forest-Products Laboratory were specially studied, 

 while more than three times this number were examined unmounted. 



TRUE TYIvOSKS 



Ray or true tyloses were found in the normal wood of the conifers, 

 but were not abundant. Their shape and general appearance are well 

 illustrated in Plate LVI, figures i and 2. None of the long, saclike 

 vesicles which sometimes fill the whole tracheid lumen in the roots of 

 conifers were found. The greatest development of true tyloses was 

 found in the soft pines. In this group they were better developed in 

 spring wood than in summer wood and were more numerous in the sap- 

 wood than in the heartwood. Indeed, some of the pit membranes in the 

 heartwood were concave in shape, appearing to have collapsed inward 

 instead of protruding into the tracheid. 



The size of the pits between the medullary ray cells and the tracheids 

 in conifers bears a definite relation to the formation of tyloses. As a 

 rule, the ray pits in the hard pines are small and tyloses are lacking, 



