PLANT ASSOCIATIONS AND HABITATS. aT 
lichens, especially on northward-facing exposures, where the effect of 
prolonged drought is least felt, and where, as a matter of fact, a very 
large percentage of the lichens occurring on Tumamoc Hill are found. 
The great number of lichens that almost cover the surface of small detached 
rocks on north slopes near the Laboratory seem to point to the atmos- 
phere rather than to the substratum as the source of water-supply, but 
the problem is worthy of much more thorough study than it has yet 
received. 

(10) CULTIVATED GROUNDS, WASTE PLACES, AND ROADSIDES; 
MISCELLANEOUS INTRODUCED SPECIES. 
Within the limits of Tumamoc Hill and the adjacent flood-plain the 
introduction of weeds and of various cultivated species has gone on until 
these have become, in many places, a conspicuous and more or less impor- 
tant element of the vegetation. The high mallow (Malva parviflora) and 
cocklebur (Xanthium canadense), for example, have greatly modified the 
aspect of roadsides and edges of fields. Along the arroyos Arundinaria, 
and near the old mill quince-bushes and Sapindus, are permanent re- 
minders of the planting of earlier days. Among later arrivals are the 
alfilaria (Erodium cicutarvum) and foxtail (Hordeum murinum), both of 
which have made themselves at home, and their presence is materially 
felt over wide areas. 
The physiological requirements of these and numerous other species 
introduced here, as well as their distribution in relation to local condi- 
tions, are, to all appearances, much the same as in the different regions 
from which they came, but it is noteworthy that they exhibit comparative 
indifference to intense insolation, much like indigenous species already 
referred to that commonly grow in shady places. With the one group 
as with the other, a sufficient supply of soil-water is evidently the first 
essential, and, with this assured, they flourish in the fierce glare of the 
sun in an atmosphere often of very low relative humidity. 
