BOTANICAL GAZETTE 227 



two kinds of wood are mentioned which have a lower specific gravity than 

 Kicinus, namely, Sali.r discolor, 0.2259, and Yucca baccata, 0.2724. It seems wor- 

 thy of note that a solid trunk of wood, three inches in diameter, even of no 

 better quality than this, was ( grown from the seed in seven months. — A. N. 

 Prentiss, Cornell University. 



The Shapes of Leaves. — Following is a resume of a series of papers in 

 Nature on the above topic, by Prof. Grant Allen. Like his other recent contri- 

 butions to evolutionary botany, they contain the result of much careful observ- 

 ation and clever reasoning ; and while we can not give assent to all his positions, 

 we welcome the essays because they can not fail to stimulate inquiry in this 

 much-neglected field. 



I. General Principles. — The leaf is the essential and really active part of the 

 vegetable organism. Its chief function is the absorption of carbonic dioxide from 

 the air, and its deoxidation under the influence of sunlight. Two main con- 

 ditions affect the shape and size of leaves: first, the nature and amount of the 

 supply of C0 2 ; and second, the nature and amount of the supply of sunshine. 

 There is a great struggle among plants for the C0 2 of the air, and through nat- 

 ural selection each plant tends to have its chlorophyll disposed in the most 

 economical way for catching such sunlight as it can secure, and its absorbent 

 surface so disposed as to catch 'such particles of carbon as pass its way. Each 

 plant inherits a general type of foliage from its ancestors, and modifies it to suit 

 the exigencies of its altered conditions. The actual shape is not always the 

 ideally-best shape for those conditions, but it is the best possible adaptive 

 modification of a pre-existing hereditary type. The venation tends most gen- 

 erally to reproduce itself under all varieties of external configuration. This 

 venation is a fixed generic or tribal characteristic, and with very slight structu- 

 ral modifications we find great differences in the resulting outline. Banuneu- 

 lusaquatilis has two forms of leaves, those floating on the surface full and round- 

 ed, the lower ones, like all submerged leaves, becoming minutely subdivided 

 into long, almost hair-like filaments, in this simulating the streaming Algw and 

 Characea. Both forms of leaf preserve the ranunculaccous type in their ve- 

 nation. 



II. Extreme and Intermediate Types.— Where access to sunlight and CO L , is 

 unimpeded in all directions the leaves tend to assume a completely rounded 

 form. This condition is most common on the surface of the water, hence most 

 water plants with floating leaves take this shape, e. g., Lemna, Nelumbium and 

 Nymphasa. Occasionally this freedom is found among land plants, e. g., Pod- 

 ophyllum, Tropaolum majus, and Hydwcotyle. The common weedy plants, espec- 

 ially the annuals and non-bulbous perennials, which, growing thickly together, 

 can not afford material to push broad leaves above their neighbors' heads, and 

 are therefore compelled to fight among themselves for every passing particle 

 of carbon, have their leaves very minutely subdivided, e. g., the UmbeUiferce. 

 The whorl ing of linear leaves occasionally serves the same purpose as minute 

 segmentation. Sometimes plants with ovoid leaves in a rosette insure them- 



