I 



^oy- THE FALLING OF LEAVES. 68i 



autumn, but the heat from the sun being often considerable, continues 

 to raise the sap in tlie stem ; the leaf, however, is already choked, and 

 can admit no more, and so strong is the pressure of the ascending 

 current against the petiole that it falls off. Murray (3) (1785) offers 

 another mechanical explanation ; he says the pressure of the growing 

 bud in the axil of the leaf hinders more and more the flow of sap into 

 the latter, causing it to die and fall off. Gerard Vrolik (4) (1797), 

 though his explanation is based rather on analogy with animal struc- 

 tures than on observation, comes nearer the mark. Every organic 

 part has its own life-period, which it fulfils, and then dies at the 

 appointed time, independently of the duration of other parts. Pre- 

 mature death of the leaf may be brought about by excessive cold, 

 heat, or disease. The separation of dead leaves is due not to 

 mechanical causes but to the absorption of a layer between the dead 

 and living parts yet belonging to the living part. This absorption 

 occurs first in the cell-tissue, and later in the vascular bundle of the 

 leaf-joint. Vaucher (8) (1821) imagined a parenchymatous tissue 

 between the stem and leaf which, so long as it was living and succu- 

 lent, strongly connected the two members. In the autumn it dried 

 up or altered in such a way that the union was interrupted. He also 

 believed that the fibres did not run uninterruptedly from the stem into 

 the leaf, but were connected by a suture formed most probably of 

 parenchyma in which the separation occurs when the fibres in the 

 petiole die. Schultz (9) (1823) thought that the ends of the members 

 of which he imagined the vessels composed were on a level at the 

 place of separation, and that they became gradually shut off from 

 each other, so that at last no flow of sap from stem to leaf was 

 possible. He does not explain how the actual separation took place. 

 Link (6) (1812) suggested that the direction of the cells was altered at 

 the place where the separation ultimately took place. According to 

 Treviranus (11) (1835) also, the cell-tissue at the leaf-joint differs from 

 that of the rest of the organ in form, size, and direction ; and when the 

 vigour of the life-processes diminish, there is insufficient force to push 

 the sap through the joint and the different cell-masses become 

 separated. Schacht (13) (1859) makes leaf- fall the result of the 

 stoppage of the flow of sap between stem and leaf by the gradual 

 formation of a cork-layer across the joint. 



Mettenius (12) (1856), who really seems to have made careful 

 observations, states that in those ferns which throw off their leaves, 

 the process is furthered and made possible by the death of a layer of 

 delicate-walle(j parenchyma, which, in the same way as in dico- 

 tyledons, is formed between the leaf-cushion and the petiole. 



Dr. Inman (14), in a short paper read before the Literary and 

 Philosophical Society of Liverpool in March, 1848, describes the 

 gradual formation of a "process -of bark" across the leaf-joint, and 

 also states " that the disruption takes place invariably through the 

 cellular tissue external to the prolongation of the epidermis " or 



