sin J. LL'BBOCK— niTTOCIOLOGICAL OBSERVATIONS. 313 



expose as large a surface as may be to the action of the water. 

 We know tliat tlie gills of fish consist of a number of thin plates, 

 which, while in water, float apart, but have not sufficient con- 

 sistence to support even their own weight, much less any ex- 

 ternal force, and consequently collapse in air. The same thing 

 happens with thin, finely-cut leaves. In still water they afford 

 the greatest possible extent oE surface with the least expenditure 

 of eftbrt in the formation of sla'leton. This is, I believe, the ex- 

 planation of the prevalence of tliis form in subaqueous leaves, as, 

 for instance, in Mijriophijllum, Ilottonia, UtricuJaria, Eanunculus, 



and many others. 



Again, in still air, the conditions, except so far as they arc 

 modified by the weight, would approximate to those of water; 

 but the more the plant is exposed to wind the more w^ould it 

 require strengthening. Hence, perhaps, the fact that herbs so 

 much oftener have finely-cut leaves than is the case with trees. 

 In the Umbellifers, for instance, almost all the species have the 

 leaves much divided, more, I need hardly say, than is the case 

 with trees. Shrubs and trees are characterized by more or less 

 entire leaves, such as those of the Laurel, Beech, Ilorubcam, 

 Lime; or by similarly- shaped leaflets, as in the Ash, Horse- 

 chestnut, and AValnut. 



There are, however, many groups of plants which, while 

 habitually herbaceous, contain some shrubby species, or vice versd. 

 Let us take some groups of this description, in which the herba- 

 ceous species have their leaves much cut up, and see what is the 

 character of the folinge in the shrubby species. 



The vast majority of Uinbellifers, as I have just observed, are 

 herbaceous and witli leaves much divided, the common Carrot 

 being a typical example. One European species, however {Bu- 

 pleurum fruticosum), is a shrub attaining a height of more than 

 G feet, and has the leaves coriaceous and oblong-lanceolate. 



Most of our common Groundsels (^«?^e/o), again, are low herbs 

 with much-cut leaves. Some species of Senecio, however, are 

 shrubby, and their leaves assume a totally diff"erent character— 

 8enecio \aurifoVms and S_, populifoHus having, as their specific 

 names denote, leaves respectively resembling the Laurel and 

 Poplar. In the genus OxaUs, again, there is a shrubby species 

 0. LaureoU, with leaves like those of a Laurel. 

 ' I would venture, then, to suggest that this throws light on the 

 reason why herbaceous plants so often have their leaves cut up. 



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