46 PLANT SOCIOLOOY 



Erica arhorea, and Cistus sahifolius, which are suppressed by the shade 

 of trees, spring up, flower, and fruit luxuriantly as soon as the forest is 

 cut. Many shade species, however, thrive badly when deprived of 

 shade. 



In deciding the sociological affinities of a species the vitalitj^ must be 

 considered, and for an understanding of the ecology of the community 

 it is necessary to know whether each constituent carries out its life 

 cycle regularly and efficiently or just barely exists. 



A seed plant is evidently at home only when it can produce flowers 

 and fruit. Reduced vitality indicates, therefore, that the optimum of 

 conditions for the species are either not attained or are overstepped. 

 Many cryptogams which reproduce wholly asexually are exceptions. 

 Yet even with cryptogamic communities vitality plays a definite 

 role. Hayr^n (1914) describes a Lecanora maura community of the sea 

 cliffs at Tvarminne in Finland, which is confined to the north or shaded 

 side of a fissure, where L. maura occurs abundantly fruiting. On spots 

 exposed to light the lichen grows but is sterile and depauperate. 

 Many aquatic plants, low shrubs, and rhizome geophytes survive over 

 large portions of their range without ever fruiting, as has often been 

 reported by explorers in high mountains and in polar regions. 



The trend of development of a community is often first indicated by 

 changes in the vitality of the species, and conversely, feebly growing 

 remnants indicate the earlier stages of development now outgrown by 

 the community, e.g., dying Carex elata tussocks in the Molinietum 

 caricetosum hostianae. 



Degrees of Vitality. — For expressing the vitality of the species in a 

 community the following four grades and signs are used : 



Well developed, regularly completing the life cycle (•). 



Strong and increasing but usually not completing the life cycle (many 

 mosses) (O). 



Feeble but spreading, never completing the life cycle ( © ) ; 



Occasionally germinating but not increasing; many ephemeral adventive 

 plants (O)- 



In studies of vegetation in which it is not necessary to consider 

 vitahty minutely it is still desirable to distinguish the species with 

 reduced vitality and feeble growth. These may be indicated by a 

 zero following the sociability grade as an exponential vitality sign 

 (Soc. 2"). 



With parasitic fungi vitality expresses itself as power of infection. 

 Hammarlund (Hereditas 6 : 1925) has shown that the spores of various 

 Erysiphaceae which germinate in the first 24 hr. have much greater 

 power of infection than those germinating later. The vitality can be 



