piiTurriNE WOODS. 355 
(Dillenia spp.)^ etc.^ and, in otliors^ so small as to be invisible witliout 
tlie aid of a magnifying glass, as in acle {PUhccolohiutn acle (Blco.) 
Vid.), betis {Illipe hetis (Blco.) Merr.), eamagon (Diospyros spp.)? or 
banaha {Lagcrstj'ccinia sprrio,<ia (L.) Pers.). The pith-rays may be all 
of the same size in the same tree, or there may be some large ones (the 
primary pith-rays) running from pith tx> bark, and some finer (secondary 
rays) starting beyond the 2>ith. Conipouiid pitli-rays, where several are 
crowded together, may also ocenr. Pitli-rays may take eitlier a crooked 
or a straight course from the pith to the bark, but -if curved, they usually 
are not abruptly so. Tlie height of pith-rays is varial)le; they nuiy be so 
short as scarcely to appear to have this dimension, or again it may he 
quite appreciable. 
Growth rings. — The wood is formed in layers about the ])itb; and 
these mav be formed onlv durin^ir certain seasons, the tree restinjr the 
remainder of the time. Where this is the cajie, each ])eriod of growth 
produces a ring about the pith. These rings are found in nearly all 
woods of temperate regions and in some of those of the Tropics. Where 
but one of them is formed during the year, it is called an annual ring, 
but manifestly, tliis name is not suitable for use with onr M'oods, since 
we do not know whether one year sees the growth of one or of several. 
Consequently, the term annular, or seasonal growth rings has seemed 
preferable and will be used in this pajier. 
Seasonal rings seem to be characteristic of some of our woods only. 
It seems probable that the same species may have them when grown under 
one set of conditions and not imder difTorent ones. It also a2)pear8 that 
many trees exhibit rings of seasonal growth when they are young but 
not afterwards. We have begun, in cooperation with the Bureau of 
Forestry, a series of observations on the manner and rate of formation of 
groMth rings, but it will necessarily be some years before any safe general 
conclusions can be reached. 
Distinct seasonal rings seem to he of constant occurrence in narra 
{Ptcrocarpm spp.), banaba {Lagerstroemta spcdosa (L.) Pers.), calan- 
tas (Toona spp.), ipil {Intsia s])p.), supa (Sindora snpa Jlerr.), molave 
{Vitex spp.), and several other woods, but there seems to be a consider- 
ably greater number where they are not so. 
False seu^onal rings, — A nundjer of woods show distinct, concentric 
linos bearing a strong, sup<Tficial resend)lance to seasonal rings. ^I'liese 
false rings may be caused by lines of soft tissue, as is the case in dita 
(Ahlonia scJiuJaris B. Br.) and palo maria {Caloplnjllani spp.), where 
they are so close together as to make it unlikely that they will often be 
mistaken. Lines of whitish resin-canals often give the appearance of 
seasonal rings in lauan (SJiorca spp.), apitong (Dipfcrocarpus spp.), 
yacal {Ilopea spp.), guijo (Shorca guiso Bl.), etc. These may readily 
be distinguished from the true seasonal rings by their irregularity of 
occurrence and by the fact that they usually fade out before conipletely 
