414 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



The " skeleton " of a dicotyledonous leaf shows how the fibres branch 

 in order to support the intervening chlorophyllous tissue and display it 

 to the light with the greatest advantage. As a general rule the more 

 leaves grow naturally, well exposed and having plenty of water, the broader 

 they are in comparison with others. If, however, plants habitually grow 

 in very dry places the leaves become greatly reduced. The result is they 

 assume the "ericoidal," or heath-like, type, as in South African Pimelias ; 

 or they may be of the box-leaf form, as in alpine Veronicas of New 

 Zealand; or minute, as Lycopodium ; or, again, there are the "needle" 

 form of conifers, or the Thuja type ; or they may vanish altogether, as 

 in the broom. When this is the case the green stem undertakes the 

 functions of leaves, as it does in Cactaceae, fleshy Euphorbias, and 

 Stapelias, in many of which the leaves are reduced to spines ; but it must 

 be borne in mind that spiny stems, leaves, bracts, &c, do not imply any 

 intention on the part of nature that they should act as protections against 

 browsing animals, for they are simply inevitable results of drought, or 

 xerophytic conditions, to which the plant cannot help responding. 



On the other hand submerged aquatic leaves are also degenerated 

 from an opposite cause. Among dicotyledons, which have become 

 aquatic from an ancestral terrestrial habit, as of buttercups, the blade, 

 still retaining the radiating distribution of the skeletal fibres, has the 

 interstitial chlorophyllous layer totally arrested ; so that the blade is 

 described as "dissected," as in the water-crowfoot. This has been 

 proved to be due to the inability of the protoplasm of the stem to 

 form a complete blade, because of its supersaturation by water, and it has 

 thus become too weak to develop the leaf. This difficulty can be, and 

 has been artificially, by Mr. MacCallum, of the United States, over- 

 come by rendering the water denser with nutritive salts. Osmotic 

 action was set up, so that the protoplasm of the stem recovered its 

 normal strength, and then the subsequent leaves formed under water were 

 complete and not dissected. The plant he employed was Proserpinaca 

 palustris, which has complete leaves in the air, but dissected ones when 

 submerged. 



Monocotyledons with parallel venation in their so-called " leaves," 

 which are really homologous with petioles, retain the ribbon-form as 

 long as they are submerged, as in Sagittaria sagittifolia ; but when they 

 approach the surface a blade may be formed at the top. 



A broad petiole thus acting as a leaf is called a phyllode. In the 

 case of the Australian Acacias the blade is seen in the seedlings, but as 

 a rule it is arrested in the later leaves. The blade is retained, however, 

 in A. melanoxylon. 



If it be asked why some leaves, as of most members of the 

 Leguminosae, have compound leaves while others have simple ones, 

 a hint is at least given by plants in which transitions and inter- 

 mediate forms occur. Thus, if a long summer-shoot of the snowberry 

 (Symphoricarptis) be examined, the first formed leaves, at the base of 

 the shoot, are small, with an oval edge ; but in the middle of the shoot 

 the leaves are very much larger, but more or less deeply indented. Then, 

 again, as the vigour declines the terminal leaves reassume the form of 

 the first. The interpretation might be suggested that the changes of 



