810 DEFOLIATION, OR THE FALL OF THE LEAF. 
be produced either chluro- or chromo-plastids, and the latter 
may also be formed from chloroplastids.. Meyer and Schmidt 
confirm these views in a remarkable manner, while Schmidt 
further suggests that there is some definite and close relation- 
ship between these plastids and nuclei. 
(7) Defoliation, or the Fall of the Leaf.—Leaves are essentially 
temporary organs ; for after a certain period, which varies in 
different plants, they either gradually wither upon the stem, 
as is usual in Monocotyledons and Cormophytes (see page 190), 
and also in some Dicotyledons (page 189) ; or they separate from 
the stem by means of an articulation when they have performed 
their active functions, or even sometimes when quite green. 
In the former case, as we have seen, the leaves are described 
as non-articulated ; in the latter, as 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. 
But the leaves of some other plants, such as Firs, Boxes, Hollies, 
frequently remain for two or more years. In the former case 
they are said to be 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 understcod. The opinion commonly entertained 
is this: the membrane constituting the walls of their cells gra- 
dually becomes so incrusted by the deposit of earthy and other 
matters which are left behind by the fluid substances which are 
contained in or transmitted through them, 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 fall 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. When it happens, it is dependent on an organic sepa- 
ration or articulation, which Asa Gray thus describes :—‘ The 
formation of the articulation is a vital process, a kind of disin- 
tegration of a transverse layer of cells, which cuts off the petiole 
by a regular line, in a pertectly uniform manner in each spe- 
cies, leaving a clean scar (fiy. 207, b, b) at the insertion. The 
solution of continuity begins at the epidermis, where a faint 
line marks the position of the future joint while the leaf is 
still young and vigorous; later, the line of demarcation be- 
comes well marked, internally as well as externally ; the disin- 
tegrating process advances 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 prolongation of the epidermis, so that when 
