10 BULLETIN" 606, U. S. DEPARTMENT OE AGRICULTURE. 



2. Pores closed by tyloses or- gums. — When the pores or vessels are 

 closed with tyloses, as in the post oak, 1 Plate IV, or with gummy 

 substances, the relations are more complicated. Tyloses, if strongly 

 developed, grow together and practically fill the vessel cavities, thus 

 stopping the penetration through these elements. Creosote may be 

 absorbed and stain the walls for a very short distance, but the main 

 lines for liquid transfer are blocked. Various effects are produced 

 by tyloses and gums according as these are more or less developed in 

 different species. If the tyloses which grow out from different points 

 on the walls of the pores or vessels do not meet and grow together, or 

 if they are weak and readily broken down, they do not effectively 

 check penetrance, as, for example, in chestnut and green ash. If they 

 do not occur in all the vessels but only here and there, as in some 

 diffuse-porous woods, their effect is, in general, proportional to their 

 occurrence. 



ARRANGEMENT OF THE PORES— RING-POROUS AND DIFFUSE-POROUS WOODS. 



A much greater variation in penetration was found in the diffuse- 

 porous than in the ring-porous species. In diffuse-porous woods 

 tyloses were not so uniformly distributed, which resulted in erratic 

 or irregular penetrations. Diffuse-porous woods also showed consid- 

 erable variation, due to gums, infiltrating substances, and cross- 

 grained structure. Examples of species exhibiting these variable 

 characteristics are silver maple, sycamore, sugar maple, largetooth 

 aspen, and red gum. 



WOOD PROSENCHYMA (FIBERS AND TRACHEIDS). 



When the pores or vessels of a wood are closed by tyloses or gums, 

 penetration of the wood prosenchyma may become of primary im- 

 portance. The cells of this tissue have closed ends. Liquids in pass- 

 ing from cell to cell must then filter through the wall itself or through 

 the thin places or pits in the cell wall. The pits are poorly developed 

 or practically lacking in some hardwood prosenchyma. It is there- 

 fore apparent that penetrance in this tissue can not take place so 

 rapidly nor extend so easily for long distances as it can in the pores 

 or vessels. (In Plates I, II, III, and IV this tissue is indicated by 

 " X.") Nevertheless, the wood prosenchyma is of considerable im- 

 portance' in relation to the penetrance and absorption obtained for 

 certain woods. Hickory is a particularly good example of a wood 

 where penetrance takes place chiefly in the wood prosenchyma. 



THE MEDULLARY RAYS AND WOOD PARENCHYMA. 



The medullary rays and other parenchyma cells appeared to be of 

 little practical importance in the penetration of the hardwoods with 



1 This species is similar in structure to white oak. 



