: 
4 
3 
e 
4 
; 
ON LEAVES. 89 
sixth. Thus, in all these combinations, whether it be from the 
distance of the systems, or of the part, or from the size of the 
leaves, which diminish as they ascend, we soon find that all 
leaves are so arranged as to enjoy the free action of the solar 
light.” 
ae most plants, when the leaves have ‘cacitssins their physio- — 
logical functions, they fall, even in the year which witnesses their — 
birth. But there are others which are not detached till the 
following year, while others still remain for many years attached 
to the stem. The leaves of most of the conifers, those of the Box 
(Buxus), the Holly (Ilex), of the Orange-trees (Citrus), do not 
shed them in the year in which they are developed, but are met 
by a continual growth of new leaves. These plants are never seen. 
naked ; they constitute the plants commonly known as evergreens. 
In the first of these states the plant is said to be deciduous, from 
dz, down, and cado, I fall, when the leaves fall before the next 
spring. They are marcescent, from marcesco, withering, when 
they wither before falling, as in the Oak and Beech. They are 
persistent, from persistens, remaining, standing, when they remain 
longer than a year. 
In some plants, as the Cactus, the leaves are shed almost as soon 
as they appear. These are said to be caduccans, from cado, I fall. 
The leaf thus dies like all created beings when the purpose for 
_ which it was created is accomplished. The immediate cause of 
death seems to be this: the cells of which it is composed have 
become encrusted with foreign matter, deposited during the 
processes of digestion and evaporation carried on by the organ, 
which then becomes incapable of further action. 
On the subject of the distribution of evergreen trees, Auguste 
de St. Hilaire makes the following remarks :—“ As we retire from 
the tropics, the number of evergreen trees goes on diminishing in 
rapid succession. At Porto Allegra, near latitude 30° south, I 
found in the coldest season that the trees nearly all changed their 
leaves. At San Francisco de Paula, near the Rio Grande, in 34°, 
nearly one-third of the ligneous vegetation had lost their leaves ; 
and finally, at two degrees farther south, a tenth of the trees only 
preserved their foliage. At Montpellier, the fields in winter are 
hot yet deprived of verdure; and Lisbon, Madeira, and Teneriffe 
