1914] GROOM—TRACHEID-CALIBER 301 
Of the genus Picea, P. rubra (29.5 u) is the species having the 
narrowest spring tracheids. Of the next two species, P. Brew- 
ertana (33 u) is the only American Pacific alpine (or subalpine) 
species; while P. alba (33.5 «) goes very far north, where, accord- 
ing to Mayr, it can grow on permanently icy soil. Farther south 
‘ P. alba often occurs in swamps, including occasionally sphagnum 
bogs, and frequently mingles with P. nigra (34.5 u). The remain- 
ing three species, P. sitchensis (34 u), P: Engelmanni (35 m), and 
P. pungens (38 u), grow in places where air and soil are moist. 
Thus the three species whose habitats are regularly or potentially 
xerophilous have the narrow spring tracheids, whereas the two 
Species with the widest spring tracheids show clear demands for 
more moisture. Yet this indication is weakened by P. rubra and 
P. sitchensis. The three Atlantic species (29.5-34.5 ) belong to 
the section Mortinpa; of the four Pacific species, P. Breweriana 
(33 #) is in section Omorica, while the other three (34-38 ») belong 
to Casicra, and their delicate loose cone scales seem to suggest 
that moistness of air characterizes their habitat. 
Larix shows the narrowest spring tracheids in L. americana 
(39. 5 ), which reaches arctic sites, and, when farther south (reach- 
Ing Virginia), grows in cold deep swamps, and particularly occurs 
m sphagnum swamps. Of the other two species, L. occidentalis 
(424) and L. Lyallii (43m) differ but little in width of spring 
tracheids, but the one with wider tracheids is that which is nearly 
alpine, in fact is often termed “alpine.” Thus Larix gives no clear 
Indication of a correlation between narrowness of tracheid and 
xerophily of habitat. Compared with the evergreen conifers of the 
poate habitat, the spring tracheids of Larix are usually wider, as 
might be anticipated (r). 
In connection with the question of the influence of two of the 
factors deciding the width of the spring tracheids, namely syste- 
matic affinity and habitat, some suggestive results are yielded by a 
©omparison of these widths in different species occupying the same 
habitats, 
: In the northern Atlantic region, Abies Fraseri (30.5 u) and 
Picea nigra (34.5 m) often grow together, as do Abies balsamea 
(40 n) and Picea alba (33.5). These two species of Picea, only 
