134 PLANT HABITS AND HABITATS IN THE 



their disentanglement is not without its difficulties. For example, 

 in Triodia irritans the large development of mechanical tissue, as will 

 appear below, is probably directly related to the physical and chemical 

 effects of dryness, but the position of this tissue in the leaves is such 

 that it becomes a very effective light-screen for the chlorenchyma, 

 which is not of palisade cells, of the leaf. And from the last circum- 

 stance it might be considered to be closely associated in some way 

 with the light relations of the plant — which indeed may be the case, 

 but this remains to be proved. 



Reactions to Temperature. 



The most apparent reactions of the plants to temperature have to 

 do with the recurrence of vegetative growth and of activities associated 

 with the formation of flowers and fruits. But also the general distribu- 

 tion of plants, particularly the north-south distribution, is largely 

 dependent on temperatures. Often, also, local distribution, especially 

 aspect "preference," is directly related to the same as well. Aside 

 from these well-known reactions, which are of great importance, 

 few appear to have been definitely established. Temperature is ef- 

 fective indirectly, however, in that it is directly related to the relative 

 humidity of the air and may greatly affect the moisture-content of 

 the soil as well. Slopes, therefore, which receive the most heat may 

 also be the most arid both as regards the soil and the air. The effects 

 of temperature on various chemical processes in the plant have already 

 been referred to and need not be sunamarized in this place. 



Reactions to a Small Water-supply. 



A reduction of the leaf-surface is the most noticeable effect on 

 the perennial plants of a small water-supply with its direct accom- 

 panying physical features, a low relative humidity of the air, and a 

 rapid rate of evaporation. This reaction is shown in several ways. 

 Trees are usually small and symmetrical. The "canopy" form of 

 such of the Eucalyptus species as have a facultative "mallee" habit 

 more especially, of which E. oleosa is an example, illustrate this, al- 

 though it can be seen at every turn in the drier portions of South Aus- 

 tralia. Where the species grow by stream ways the habit of growth is 

 usually noticeably less compact. Often, usually in the drier regions, 

 the foliage is confined to the tips of the branches, by which the 

 canopy effect is heightened. The immediate effect on the plants, 

 however, is to limit the growth of the leaves or to bring about such a 

 modification of the chorophyll-bearing organs as is to be witnessed in 

 many acacias, or to wholly suppress the formation of leaves in mature 

 plants. During this process the foliar organ apparently undergoes 

 an alteration in form, so that it becomes relatively long as compared 

 with what may be supposed to be the ancestral condition, or that 

 of plants more favorably situated as regards the water-supply. This 

 observation was directly confirmed by numerous measurements and 



