of one strain is a function of others coexisting with it.^^^ However, 

 in plants the influence is usually less direct than in animals. 

 Trees, for example, may influence the ground vegetation by 

 absorbing nutrients deep in the soil and depositing them by leaf 

 fall on the surface. The effect may be specific, as shown by the 

 luxuriant growth of nettle under alder in Great Britain, while 

 under pines bracken communities develop, and under larch the 

 bramble is dominant. '^^ 



Excretions and products of decomposition, by preventing the 

 growth of some species and favoring that of others, may influence 

 the formation or maintenance of certain plant groupings. Sub- 

 stances such as biotin, thiamine, vitamin B12, histidine, and 

 uracil may be stimulative or even essential for some species. In- 

 hibitory substances include penicillin produced by Pemcillium 

 notatum, glytoxin by the soil fungus Glocladium, absinthin by 

 Artemisia absinthium, 3-acetyl-6-methoxy-benzaldehyde by the 

 desert plant Encelia farinosa, juglone by Juglans nigra, and cin- 

 namic acid by Parthenium argentatumJ^-^^ Species of sunflower 

 differ in their sensitivity to toxins given oflTby grasses, which affect 

 the association of these species according to the kinds and con- 

 centrations of the toxins and the tolerances of various kinds of sun- 

 flowers.^ It appears also that alkaloids, leached from leaves or 

 roots, may have important ecological influences. '^^ 



Although many details regarding interactions are not well 

 understood, it is clear that the requirements of species or ecotypes 

 and their ecological amplitudes play essential roles in independ- 

 ent association and in positive and negative correlation in the dis- 

 tribution of species. 



Differences of Species in Reproductive Processes 



The adaptability of plants for pollination; the production, dis- 

 persal, and germination of seeds; and vegetative propagation are 

 considered in this section. Wind-pollinated plants depend for 

 success upon the production of large quantities of pollen, while 

 insect-pollinated ones depend upon special structures, colors, or 

 fragrance. Important factors in pollination include the quantity 

 and viability of pollen produced, distance the pollen is carried, 



Ecological Characteristics of Species & Populations • 37 



