BY SELF-ADAPTATION TO THE ENVIRONMENT. 251 
corroborated on quite independent grounds by M. Hackel *, who 
has called attention to similar structures and their usage in 
certain grasses of dry climates; these being singular develop- 
ments of the lowest internodes of the culms, shoots, and basal 
leaves. 
He distinguishes them as tuberous or bulbous grasses and 
tunicated grasses. Tuberous grasses are such as Phleum 
pratense, var. nodosum, Gaud., and Arrhenatherum avenaceum, 
var. nodosum (Avena nodosa, L.), of which one or more of the 
basal internodes of the culm and shoots attain a tuberous deve- 
lopment; while Poa bulbosa, L., represents a bulbous grass, 
since the bases of some of the sheaths of the leaves have in- 
creased in thickness and form a bulb very much like that of 
Allium+. He observes that these tuberous and bulbous forms 
only occur in countries with periods of dry seasons, and none 
have been observed in the moist parts of tropical regions. 
It is very interesting to learn that the author does not consider 
these tubers and bulbs to be reservoirs of starch or sugar, as are 
the similar organs of Liliacee, Iridaceew, &c., though being 
structurally homologous with these; but physiologically they are 
water-reservoirs, just as Dr. Volkens maintains with regard to 
the tuberous roots of species of Erodium in the deserts of 
Africa. 
What is particularly to be noticed is that Hackel has shown 
that Poa bulbosa, on being cultivated in moist soil, almost 
lost the bulbous character, clearly proving therefore that these 
productions are the direct result of a dry environment. 
These tuberous swellings on grasses are therefore clearly 
analogous with those on the roots of Erodium. Thus E. hirtum 
has globular, potato-like tubercles; H. Hussoni, finger-shaped ; 
and long spindle-like roots occur in E. glaucophyllum. 
They all contain a storage-tissue protected externally by a 
strong, many-layered cortical coating. Their position being be- 
tween the absorbing root-apices and the foliar transpiring surfaces, 
they act as reservoirs and regulate the supply of water. 
Bulbs of species of Allium, as A. Cramert, &c., are similarly 
adapted to the desert, storing water within the inner scales, 
* “Ueber einige Eigenthuemlichkeiten der Graeser trockener Klimate,” 
Verhandlungen der k. k. zool.-botan, Gesellsch. Wien, Jahrgang 1890, p, 125. 
+ Dr. Aitchison observed Poa bulbosa to be “ the most common grass covering 
the great plains in Beluchistan ” (J. c. p. 432) [supra, p. 221]. 
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