154 LEAF- AERATION IN NATAL PLANTS. 



Unfortunately, many, particularly sclerophyllous and parallel- 

 veined, leaves will not inject completely. In order to prevent 

 absorption of the injection-fluid by the cells of the leaf it is 

 •essential to deal only with leaves which have been kept for several 

 hours in an atmosphere saturated with water-vapour : this can be 

 done by placing leafy twigs in water under a bell- jar for some 

 hours before experimenting. 



Several interesting results have emerged from the work already 

 done. In the first place a criticism of the conclusions of previous 

 writers (e.g. Haberlandt, McLean) seems to be justified. Evidence 

 has been obtained that a well-developed intercellular space system 

 is not necessarily a mesophytic character, nor a poorly-developed 

 one necessarily a xerophytic character. Two Natal xerophytes — 

 Portulacaria afra, the Spekboom, and A vicennia officinalis, a Man- 

 grove — have, for instance, a larger intercellular space system, rela- 

 tive to both weight and volume of the whole leaf, than species 

 growing in more "mesophytic" situations, e.g., Fagara davyi and 

 Tricalysia lanceolata in High Veld Bush. This indicates that 

 another factor, besides the water factor, has an influence on the 

 degree of leaf-aeration; and Prof. Bews has suggested that, since 

 Portulacaria and A vicennia both have a badly-aerated substratum 

 whereas the other species named have not, the aeration system of 

 the leaf is also influenced by the degree of soil-aeration. 



Careful experiments indicate that the supposed difference 

 between sun and shade leaves (of the same species) is really due 

 to an increase in the specific gravity of the leaf-substance of sun 

 leaves as compared with that of shade leaves. All previous work- 

 ers have expressed their results in terms of the live weight of the 

 leaf, and thus have obtained an apparent increase in the inter- 

 cellular space content of shade leaves; but when it is expressed in 

 terms of leaf-volume, there is no such increase. 



From the ecological viewpoint, the degree of variation 

 exhibited by various types has provided some most interesting 

 data. Among the species dealt with, those with the greatest varia- 

 bility in the magnitude of the intercellular space system are 

 species which are pioneers in plant succession (e.g., Portulacaria 

 afra, Gymnosporia buxifolia, Combretum spp.), or else are variable 

 in their ecology {Ptaeroxylon utile). Those with a less degree of 

 variability are species confined to later stages in plant succession 

 (e.g., trees in High Veld Bush) or to specialized habitats (e.g.. 

 Piper capense, A vicennia officinalis). The occurrence of such 

 variations in the aeration system of plants is of considerable 

 interest, since it is in the iutercellular spaces of the leaves that 

 gaseous exchange between the plant and the atmosphere takes 

 place. The degree of leaf-aeration, therefore, is a factor affecting 

 the three vital functions of transpiration, assimilation, and 

 respiration. Variations in so important a factor are of especial 

 significance when viewed in the light of an hypothesis to be sug- 

 gested in the next paragraph. 



The quantitative results given above are valuable in that they 

 illustrate and confirm an important ecological principle which was 



