BIOLOGY. 323 



WHY THE LEAVES FALL. 



According to Von Mohl, '* Shortly before the fall of the leaf 

 tliore begins to be formed a very delicate layer of cells, the growth 

 of which is from above downward, so that, beginning from the 

 axillary side of the leaf and graduall}^ extending downward and 

 outward, nearly at right angles to the long diameter of the cells 

 of the leaf-stalk, at any rate at right angles to the plane of the 

 leaf, it effects a gradual separation between the stem and the 

 leaf as ctfectually as a knife would do. These changes are not 

 wholly due to a change of seasons from wet to dry, or from hot 

 to cold, for it not unfrequently happens that if a tree be stripped 

 of its leaves in summer, it forms during the autumn new ones, 

 which remain on the tree during the greater part of the winter, or 

 until long after the usual period." — Pop. Science Review, 



AIR-VESICLES OF THE UTRICULARI^. 



S. B. Schnetzler, in the " Annals of Natural History," publishes 

 an account of these curious appendages to the leaves. The genus 

 Utricularia consists of aquatic plants found in the stagnant waters 

 of ditches, marshes, etc. ; the leaves are submerged and furnished 

 Avith the remarkable utricles. De Candolle states that in the 

 young plant these vesicles are filled with mucilage heavier than 

 water, which, like ballast, hold the plant down at the bottom. 

 Toward the period of flowering, the leaves secrete a gas which 

 makes its way into the utricles, and drives out the mucilage by 

 raising an operculum or lid with which the utricles are furnished. 

 '1 he plant, thus provided with a multitude of air-bladders, rises 

 slowly and floats at the surface. After flowering in the air, the 

 mucilage is again secreted, and the plant sinks to the bottom to 

 ripen its seeds. After careful investigation of the morphology 

 and history of these organs, he concludes that they play the part 

 both of organs of respiration and of a hydrostatic apparatus. 

 They do not appear at a given moment and for a particular pur- 

 pose, but as a natural consequence of the anatomical structure of 

 the plant and the action of the surrounding medium. 



ON THE MUFFA OF THE SULPHUR SPRINGS OF VALDIERI. 



A paper was read on this subject by Mr. Moggridge, at the 1868 

 meetiuo^ of the British Association. 



The Baths of Valdieri are situated in a valley on the northern 

 side of the Maritime Alps, and have long been celebrated, not 

 only for the excellence of their mineral waters, but also for the 

 "Muffa," — a substance occurring in one of those waters, which, 

 while of great medicinal value, is interesting when viewed under 

 the microscope, for the vegetable, animal, and mineral produc- 



