1922| McDOUGALL—SYMBIOSIS 211 
very different, on the other hand, often are incidentally very 
serviceable to one another. The trees, for example, furnish the 
shade necessary for some of the herbaceous plants and fungi, 
while the herbaceous plants furnish a living. soil cover which 
prevents undue loss of the soil water which is needed in great 
quantities by the trees. The trees, likewise, as well as the shrubs, 
especially those near the border of the woods, serve as a windbrake 
which protects many smaller plants from the dangers of too high 
transpiration rates. 
Of very great importance from the viewpoint of social disjunctive 
symbiosis is the phenomenon of leaf fall. The primary reason for 
leaf fall, of course, is the reduction of the transpiration surface during 
the season when absorption is difficult or impossible, and the 
primary cause is desiccation, but the effect of this habit on other 
members of the community is perhaps as important as its signifi- 
cance to the deciduous plants themselves. The fallen leaves form 
an efficient cover throughout the winter season, thus greatly reduc- 
ing evaporation from herbaceous perennial plants as well as from 
the surface of the soil. The place of the fallen stems of herbaceous 
perennials in social disjunctive symbiosis is similar to that of the 
fallen leaves, as, likewise, is that of the dead bodies of annual 
plants. 
Closely connected with leaf fall are the activities that are 
concerned with the decay of the fallen leaves. These are due 
mostly to bacteria and fungi. The bacteria and fungi are regular 
members of the community, and are living in social disjunctive 
symbiosis with the higher plants. They are able to live in the 
community only as a result of the presence of the higher plants, and 
they render a distinct service to the community by preventing the 
accumulation of dead bodies. 
Although the phenomena just cited all result in benefit to 
certain members of the community, it must be understood that 
symbiosis does not necessarily imply any benefit to the symbionts, 
mutual or otherwise. In antagonistic symbiosis there is often 
more harm than benefit for at least some of the symbionts, while 
in social symbiosis there may be a mere tolerance of presence 
with neither harm nor benefit resulting to any appreciable extent. 
Thus many of the species in a community such as the one under 
