THE ORIGIN AND GROWTH OF PLAN! COMMUNITIES 585 



cold or dry seasons, lose so much of their vegetative parts that the 

 total weight carried, the total space occupied, the total water utilized, 

 the total nutrient use and storage, and the total light and heat inter- 

 cepted vary by huge amounts. 



1. Vegetative periodicity. The response of unfolding, spreading, 

 and shedding of vegetative parts is therefore structurally as well as 

 physiologically important. The number of evergreen species, and above 

 all their total bulk, in any community is therefore of great significance. 

 A New Zealand beech forest contains practically no deciduous species, 

 whereas a New England beech forest contains almost no evergreens. In 

 the group of 14 stands of the St. Lawrence Valley, only one, the 

 Thujetum occidentalis, is predominantly evergreen, although the Acere- 

 tum saccharophori tsugosum presents a secondary character of ever- 

 greenness. Table VIII shows the exact distribution of evergreenness. 

 The disturbed maple forest and the bluegrass meadow have no 

 evergreens at all; the beech-maple-hemlock stand, the cedar forest, 

 the steeple-bush steppe, and the wire-birch forest have quite high 

 percentages. And here again (see column B) the use of a coverage 

 coefficient conveys a more accurate picture. 



2. Reproductive cycle. Likewise, the reproductive and dispersive 

 processes will give us valuable clues to the inner mechanics of the 

 stand. For instance, if reliable data were available, it would be interest- 

 ing to determine the incidence, among the species present in a stand, 

 of the following: (a) predominance of vegetative over sexual propaga- 

 tion; (b) the systems of reproduction— sexual, vegetative (obligate), 

 viviparous, or agamospermous (through adventitious embryony, agos- 

 pory, diplospor}' ) . A tentative analysis of nine stands of North Ameri- 

 can vegetation gave some indication of the results to be sought in this 

 respect. These are shown in Table IX. It appears that the open com- 

 munities show a much higher percentage of maintenance by seeding. 

 However, although about half the species of the forests owe their pres- 

 ent position mostly to vegetative propagation, very few of them are 

 either apomictic or obligately vegetative. 



3. Dispersal. It is agreed that morphological devices do not neces- 

 sarily indicate the means of dispersal. The latter must be actually ob- 

 served in situ in order to be reliably recorded as efficient. Nevertheless, 

 a number of morphological structures ( winged appendices, plumose or 

 hooked fruits ) are such highly differentiated elaborations, as compared 

 to the widespread seed diaspores of medium size and weight, that it is 

 difficult to deny their significance if they show a degree of incidence in 

 a community. A grading devised by Dansereau and Lems (1957) has 

 been applied to the 14 Laurentian stands, and the resulting unevenness 

 of distribution is quite characteristic ( Table X ) . Thus, the auxochores 

 are really high only in the forest stands, and so are the pterochores. 



