DR. K. LEDEGANCK ON THE FALL OP LEAVES IN AUTUMN. 107 



stead of forming the axis of the pulvinus, passes inside, and 

 directs itself towards the apex of the axillary ; bud. Thus the 

 pulvinus presents: — 1st. The epiderm, either in a continuous 

 layer or in strips, in the form of epidermal scales. 2nd. One or 

 more coats of suberous or corky tissue, which form the periderm, 

 the growth of which causes the destruction of the epiderm. 

 3rd. The cellular layer, the cells of which are filled with liquids, 

 and generally provided with chlorophyll (herbaceous tissue of 

 Duhamel). Two or three rows of cells on the outside are often 

 filled with a red colouring matter. 4th. The central parenchyma 

 equally full of juices, and which gives place to, 5th. Liber cells and 

 vascular bundles, which run into the leaf. 



The base of the petiole, which adapts itself to the surface of the 

 pulvinus, presents much simpler organization. On proceeding 

 from without to within we find — 1st, Epiderm, like that on the 

 surface of the leaf, with lengthened cells, like the ribs ; it remains 

 intact during the existence of the leaf, and corresponds to the 

 epidermis of the pulvinus. 2nd, Parenchyma, or prolongation of 

 the mesophyll of the leaf. This is much more abundant, looser, 

 and more porous (lacuneux) on the outside of the petiole (inferior 

 surface of the leaf J than on the interior (superior surfaces). It cor- 

 responds to these tissues. Periderm red and green cellular, and 

 central parenchyma of the pulvinus. As a whole the 

 parenchyma of the leaf stalk shows much larger cells, and 

 oftener filled with air, than the corresponding portion of the stem. 

 3rd, Vascular bundles passing the parenchyma, and crowded the 

 more, the closer we examine the base of the petiole. 



In this comparative study of the structure of the petiole and 

 pulvinus, three essential facts are brought out. 1st. — That the 

 suberous layers (periderm)of the pulvinus are not continued into the 

 leaf stalk, and stop when level with the imaginary line of articula- 

 tion. 2nd. — The cellular tissue, with chlorophyll, stops also at the 

 same level, and altogether the tissues of the pulvinus, which 

 correspond to the mesophyll of the petiole, are distinguishable from 

 the last, both by their texture and by the character of their contents. 

 3rd. — The vascular bundles are the only anatomical characters 

 common to the two organs, and which pass from one to the other 

 with perfect continuity. 



We may add that the continuity of these two epidermides is 

 quickly destroyed by course of development of the peridermal 



