FUNCTIONS OF THE PETIOLE. 



53 



pajison of the same. Ettingshausen has in the latter sense described thousands of 

 leaf-venations, without arriving at any important result whatever; while our simple 

 principle, according to which the leaf-venation fulfils, on the one hand, the carrying to 

 and fro of nutritive matter, and, on the other, the mechanical office of keeping the 

 lamina tighdy extended and protecting it from rupture, affords a very clear insight 

 into the variety of forms here prevalent. 



As regards the mechanical arrangements, however, we have yet to refer to one 

 point hitherto not touched upon. It is quite a common phenomenon that small 

 narrow leaves are situated immediately upon the shoot-axis ; 

 while the majority of large, thin leaves of aerial plants, as well 

 as of aquatics, are fastened on more or less long and thin 

 stalks. I see in this disposition a further expression of the 

 mechanical principle in the organization of the leaves : for 

 it is at once obvious, that the described arrangements of the 

 venation against rupture by the wind, or the wave-shock of 

 water, are necessarily aided when the large thin leaf-surfaces 

 swing on elastic movable stalks. Storm and wave-shock 

 hardly affect the lamina when it is situated on an elastic 

 stalk which yields readily to the pressure of the wind 

 or water current, and turns so that the leaf resembles a 

 weather-cock rather than a common flag. If, on the other 

 hand, the leaf is situated on the axis without a stalk, all 

 depends upon whether the axis itself is flexible and elastic. 

 If we now reflect, that an extremely important feature in the 

 general appearance of plants consists in whether their leaves 

 are small or large, whether they are entire or divided, whether 



they are stalked or sessile, &c., it is obvious that we obtain by means of the 

 mechanical principle here expounded, a right understanding of one of the most 

 striking and widely extended phenomena of the vegetable world. And if we ask, 

 finally, why it is just in the case of the foliage-leaves that this mechanical principle 

 is so clearly manifested, the answer is already given above : it is simply a matter of 

 presenting the chlorophyll cell-tissue of the plant to the light in the form of a very 

 thin lamina, and at the same time of providing that by means of a regulated 

 evaporation of water out of this lamina the food-materiak taken up by the root 

 also flow to the leaf ^ 



' In accordance with the purpose of the present book, I must refrain from the treatment of the 

 venation of leaves more in detail, although there is much temptation to attempt to refer these 

 relations of organization, so obvious but hitherto not understood, to a principle so simple as 

 that indicated in the text. Schwendener (D(7s mechanische Princip ini anat. Bau der Monocoiylen. 

 Leipzig, 1879) has also concerned himself, it is true, with the rigidity of leaves, so far as it is based 

 on the anatomical structure of the transverse section, but has scarcely noticed the relations pointed 

 out by me in the text. Compare also Haberlandt in Jahrb. fur wiss. Bot. Bd. XIII. pp. i6o, &c. 

 I hope later to establish further the short notices given in the text, in a detailed exposition. 



