83 
hygroscopic movement, the pods possess no explosive power, as 
in the case of members of the genus Phaseolus. 
The awns of many grasses, notably those of the S¢ipas and 
Andropogons, twist and untwist with changes of moisture, but 
those of Hordeum jubatum, ( Figs. 22, 23), show very marked 
movements pointing towards the performance of quite a different 
office. While the movements of the awns of the Andropogons 
probably facilitate the entrance of the seed into the ground, those 
of the “ Squirrel-tail”’ simply serve to separate joint from joint of 
the dry spike, and aid in their transportation from place to place, 
like a tumble weed. The movement is simply a lateral straight- 
ening or backward curving of the awns, which, wedge-like, raises 
the spikelet from among those below. Moisture has the effect of 
quickly reversing the process. 
Similar to the Jast are the movements made by the pappus of 
numerous species of Composite. A sprig of Solidago Canadensis, 
some days past blooming, was placed among others in a glass of 
water and in five minutes the involucres, which were open as at 
Fig. 24, had closed in upon the achenia and pappi so completely 
as to make it almost a matter of doubt if the sprig was the same 
(Fig. 25), Upon exposure to the dry atmosphere for an hour or so, 
the involucres opened, and the achenia were soon seen to separate 
from the receptacle and mount upon each other by the force of 
the expanding pappus. 
The heads of Brunella vulgaris are susceptible in a high 
degree to the changes of moisture. In dry weather the calices 
with their short pedicels stand closely appressed to the main 
stem, (Fig. 26), while in wet weather, they bend outward and 
even downward, (Fig. 27), giving the head a loose, bushy appear— 
ance quite different from its aspect in a dry atmosphere. 
Upon examining the heads of the common Bergamot ( Mo- 
narda fistulosa), it was found that the dry, tubular calices, which 
are arranged on branches or arms radiating from the center, are 
almost always split open at their bases (Fig. 28). At first this 
almost universal splitting of the calyx, together with the pres- 
ence of the chinks in the floor of the head, were taken as indicating 
a regular mode of seed distribution ; but further examination 
showed it to be only a secondary mode at most; the primary 
