684 NATURAL SCIENCE. kov.. 



beginning of October ; lie could not find it in many plants on October 

 5th, e.g., ash, Ampelopsis and others, but it formed in these in the 

 course of the next five to ten days. No definite time can, however, 

 be given, as it depends on the weather, the position of the tree, and 

 also varies, as, for instance, in the poplars mentioned above, in the 

 different leaves of the same tree according to position on the twig, or 

 whether shaded or exposed to the sun. It was quite absent, or faint 

 traces only were present, in the almost completely dried leaves of the 

 beech and several species of oak in the second half of October. 



Mohl definitely states that only two rows of parenchymatous cells 

 areusedinitsformation. When the leaf nears its fall, the cells separate 

 without tearing and the free cells become rounded off; they are not 

 by any means dead, but contain a well-defined layer of protoplasm 

 and become turgid on addition of water. The transition from firm 

 union to complete separation is, moreover, not sudden, but spread over 

 a time during which the walls get weaker and are more easily 

 separable-. In none of the plants examined had tiie vascular bundles 

 running through the joint undergone any visible change. There was 

 no general rule as to the place of separation, except that it never 

 occurred in the cortex of the stem, but always in the tissue of the leaf- 

 stalk, usually in the lower region of the basal swelling. The place is 

 not previously indicated ; it appears in the hitherto perfectly uniform 

 parenchyma of the part of the leaf-stalk. 



Such is the account of the anatomical changes resulting in the 

 fall of the leaf given by the German botanist two and thirty years 

 ago. With a few modifications and additions it represents the 

 present state of our knowledge. 



In i882MM.Guignardand Van Tieghem( 19) communicated to the 

 Botanical Society of France the result of some researches conducted 

 specially on Gymnocladns canadensis — the plant of which Mohl also made 

 special use. They investigated the fall of the individual leaflets, as 

 well as of the whole leaf, and their researches, moreover, showed the 

 condition of the parts concerned as early as the middle of June. 



As regards the leaflets, the formation of the separating layer was 

 the only phenomenon observed, and that only a short time before the 

 fall. A thin layer of active tissue (secondary meristem) is formed in 

 which the cells of the epidermis, and the bast and wood parenchyma, 

 also take part, only the sieve-tubes, the vessels, and the fibres being 

 omitted. The separating layer consists of three rows of cells, of which 

 the middle one is absorbed, while the walls of the upper and lower, 

 relie\ed on one side from pressure, become rounded towards and 

 press upon each other, and the tension thus caused snapping the 

 vessels and fibres, the leaflet is literally thrown oft' by the force of the 

 turgescence of the separated cells. The wound is left exposed ; 

 there is obviously no reason for the careful protection of scars on a 

 leaf-^talk which will itself soon drop. 



As regards this common leaf-stalk, an important point is the 



