10 BULLETIN 606, U. S. DEPARTMENT OE AGRICULTURE. 
2. Pores closed oy tyloses or gmris. — 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 reel 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. 
