760 PHYSIOLOGY. 



their active functions, or even sometimes when quite green. In 

 the former case the leaves are non-articulated ; in the latter 

 articulated. In the trees of this and other temperate climates 

 the leaves commonly fall off the same year in which they are 

 developed, that is, before the winter months ; and in those of 

 warm and tropical regions the fall of the leaf often takes place 

 at the dry season. The leaves of some other plants, such as Firs 

 and Pines, however, generally remain for two or more years. In 

 the former case they are annual or deciduous, and in the latter 

 persistent or evergreen. The fall of the leaf is commonly termed 

 defoliation. 



The cause or causes which lead to the death of the leaf are by 

 no means well understood. The opinion commonly entertained 

 is this : the membrane constituting the walls of their cells gra- 

 dually becomes so incrusted by the deposit of earthy matters 

 which are left behind, that ultimately the tissues of the leaf 

 become choked up, and are no longer able to perform their proper 

 functions, and the leaf then begins to dry up. After its death 

 the leaf may either fall, or remain attached to the stem, as 

 already observed. 



The/rt?^ of the leaf does not, then, depend upon the death of 

 the organ ; it may occur before death, or may not take place at 

 all. Wlien it happens, it is dependent on an organic separation 

 or articulation which Asa Gray thus describes : — " The formation 

 of the articulation is a vital process, a kind of disintegration of 

 a transverse layer of cells, which cuts off the petiole by a regular 

 line, in a perfectly uniform manner in each species, leaving a 

 clean scar ( figs. 1 85, h and 346, /) at the insertion. The solution 

 of continuity begins at the epidermis, where a faint line marks 

 the position of tlie future joint while the leaf is still young and 

 vigorous ; later, the line of demarcation becomes well marked, 

 internally as well as externally; the disintegrating process ad- 

 vances from without inwards until it reaches the woody bundles ; 

 and the side next the stem, which is to form the surface of the 

 scar, has a layer of cells condensed into what appears like a pro- 

 longation of the epidermis, so that when the leaf separates," as 

 Inman says, " the tree does not suffer from the effect of an open 

 wound." G-ray, then quoting Inman, adds : — " The provision for 

 the separation being once complete, it requires little to effect it ; 

 a desiccation of one side of the leaf-stalk, by causing an effort 

 of torsion, will readily break through the small remains of the 

 fibro-vascular bundles ; or the increased size of the coming leaf- 

 bud will snap them ; or, if these causes are not in operation, 

 a gust of wind, a heavy shower, or even the simple weight of the 

 lamina, will be enough to disrupt the small connections and 

 send the suicidal member to the grave. Such is the history of 

 the fall of the leaf" 



8. Bevelo]pment of Leaves. — Nearly all that we know upon the 



