io8 
Peirce.— The Dissemination and 
Phoradendron one sees at once that the habit of A. occidentale at least 
is entirely different from that of members of the other two genera. In 
A. occidentale the stem is composed of very evident segments, rectangular 
or square at the base, and cylindrical at the top of each segment. The 
colour is much lighter, the abundant chlorophyll being masked by a 
brownish pigment in the walls of the epidermal cells. The brittle stem 
breaks into pieces much more readily near the nodes than near the middle 
of the internodes, in this respect resembling the grasses and other * jointed ’ 
plants. The stem branches profusely, the stems of Viscum and Phoraden- 
dron comparatively sparingly. The leaves are reduced to scales subtending 
the chlorophyll-containing branches. 
In the reduction of the leaf surface, the thickening of the outer walls 
of the epidermis, the sinking of the guard-cells of the stomata below the 
surface (described by Cannon in Phoradendron also), the screening of the 
chlorophyll by another pigment or pigments (see also Cannon), we have 
very decided differences from the Viscum album of northern and moister 
Europe, and very evident protective adaptations to conditions which, at 
certain seasons of the year at least, approach much more nearly those of 
the desert than those of Europe. Certain species of Arceuthobium are 
found in the Sierra Nevada Mountains, where evaporation into the dry air 
is very rapid in summer, and in parts of Arizona and New Mexico, which 
are really semi-desert. While A. occidentale is actively vegetating, in the 
spring, the air may be comparatively moist, and from its host it can absorb 
water in plenty as the soil under the pines is full of moisture, at least below 
the surface. But in the summer, when no rain falls and the air is very 
dry, except when fogs sweep in from the ocean, the parasite must retain 
as much water as possible lest it perish. There is a very decided contrast, 
therefore, between summer and winter conditions. While the conditions 
for germination are those of a mild and humid climate, those of vegetation 
and fruiting are those of a dry and warm climate. To these two sets of 
conditions the parasite has adapted itself. In this respect it is like Phora- 
dendron villosum, but it has carried its adaptation to the summer conditions 
further than has Phoradendron. The conditions for germination are pro- 
bably the same in both plants, for only once have I succeeded in germinating 
seeds of Phoradendron villosum , and then the young plants soon perished, 
though I have tried at least five years. 
Something more should now be said of the anatomical relations of 
parasite and host, and of the effects of the parasite upon the host. Figure % 
shows the direct connexion of a branch of Arceuthobium with the wood of 
the host. In Fig. 22 we have a diagram showing two masses (parts of 
one) of Arceuthobium cells in part of a cross-section of a pine branch. The 
tissues of the parasite are shaded, those of the host white. In this figure 
we see that strands of cells of the parasite run from the main mass in the 
