perennials {Opuntia spp.), small annuals {Festuca octoflora), and 

 various short and tall forbs. The kinds of life-forms, the number 

 of individuals of each kind, and their spacing give structure to 

 the community. 



The type of life-form has considerable influence on the asso- 

 ciation of species. In pastures in England, grasses are less favor- 

 able means of infection of sheep by worms than red and white 

 clovers because the lower blades of the grasses present more ob- 

 stacles to the worm when it is climbing up from the ground than 

 the straight leafstalks of the clovers. A stand of late-maturing 

 grasses often provides good opportunity for the growth of early- 

 maturing forbs. Head smut {Ustilago marginatus) has been found 

 to be more prevalent on a strain of Bromus marginatus character- 

 ized by early maturity and moderate leafiness, and less prevalent 

 on a late-maturing, very leafy strain. 



The general appearance of a community is caused more by the 

 life-form of the most abundant or dominant species than by any 

 other characteristic of the vegetation (Figure 1-16), so it is not 

 surprising that it has been used very widely for description. Of 

 the many classifications that have been proposed, Raunkiaer's 

 simple system 1^^ is the most widely known and used. It is based 

 upon the overwintering parts or the location of organs that sur- 

 vive summer drought or other unfavorable conditions. This 

 classification, as modified somewhat by Braun-Blanquet,^^ com- 

 prises ten main classes, as follows: 



(1) Phytoplankton: Microscopic plants suspended in air, 

 water, or on snow 



(2) Phytoedaphon: Microscopic soil flora 



(3) Endophytes: Plants living wholly or partly within other 

 plants, as algae in lichens, or parasites 



(4) Therophytes: Annuals, including algae, fungi, liverworts, 

 mosses, and many ferns and seed plants (Figure 1-7) 



(5) Hydrophytes: All water plants, except plankton, with 

 perennating parts submerged in water during unfavor- 

 able periods 



(6) Geophytes: Plants with perennating parts buried in the 

 substratum, such as species with rhizomes or bulbs (Fig- 

 ure 2-5) 



Analytic CHaracteristics of «He Coxnnmunity • 93 



