1908] HOLM—ALPINE GRAMINEAE 437 
walled mestome sheath was observed in all the alpine Gramineae, and 
these species may thus be added to the list given by SCHWENDENER®? 
in his paper on mestome sheaths. However, the presence or absence 
of the mestome sheath, as already pointed out by SCHWENDENER 
(/.¢., p. 415), is merely of taxonomic importance. This may be 
readily seen from his list, according to which this sheath is not devel- 
oped in any of the species of Andropogoneae and Maydeae, or in 
certain genera of the Paniceae (Paspalum, Pennisetum, and Setaria). 
The fact that it occurs in some species of Panicum (P. miliacewm, 
P. capillare, P. proliferum), but not in others (P. sanguinale, P 
plicatum, P. colonum, etc.), seems to indicate that these species repre- 
sent very distinct types within the genus, as shown also by the external 
structure of their spikelets. The same conclusion may be drawn from 
the fact that the species of Aristida in which I observed a double paren- 
chyma sheath,"s but no mestome sheath, differ in a marked degree 
from those which possess this sheath, and in which only a single paren- 
chyma sheath is developed; we have here to deal with a taxonomic, 
and not with an epharmonic character. By studying the anatomy 
of a number of Gramineae allied to or associated with Aristida, I 
found a mestome sheath constantly developed, whether the material 
was collected on the plains, the prairies, in woodlands, or in marshes. 
If on the other hand the structure of the mestome sheath is examined, 
some kind of modification in the thickening of the cell walls is noticed, 
which evidently constitutes an epharmonic character; in the alpine 
Species this sheath was generally observed to be quite thick-walled. 
The presence or absence of thick-walled mestome parenchyma as a 
Stratum between the leptome and the hadrome is to be considered only 
of taxonomic importance; such parenchyma was not observed in Poa 
Lettermanni, P. gracillima, P. rupicola, or in the species of Festuca 
and Avena, but in all the others. 
Of much greater interest, however, is the structure of the cortical 
Parenchyma. This tissue is very compact in these Gramineae with 
the exception only of Poa Fendleriana, P. gracillima, and Phleum 
es It is either developed as a palisade tissue of several layers 
NDENER, Die Mestomscheiden der Gramineenblatter. Sitgungsber. 
Berliner Akad. Wiss. 413. 1890. 
*3 Hotm, THEO. Some new anatomical characters for certain Gramineae. Beih. 
Bot. Centralbl. II:—. Igor. 
