62 LEAVES. 
212. Deciduous, when they fall before the new leaf a 
pears, as in the Horse-chestnut and most other trees. 
213. Marcescent, when they wither before falling, as in th 
Oak and many other trees. 
214. Persistent, or Evergreen (Sempervirens), when 
remain on the vegetable one winter or longer, as the Ivy. 
the Pine, the Myrtle, the Common Laurel, &c. Plam 
of this kind are called Evergreens. Sooner or later | 
leaves of Evergreens fall like those of other plants. 
215. In the generality of plants the leaves annually 
cay, and are reproduced in the spring. It is supposed 
before the leaf falls there is a kind of joint formed bety 
it and the stem. It is chiefly in the dicotyledonous 
that this articulation is formed between the leaf and stem 
In monocotyledonous plants and those acotyledonous 
plants which have leaves, the joint is frequently a 
The articulation is effected by the deposition of 
matter, and by the drying and shrinking of their 
at the junction with the stem. In very warm cli 
their decay occurs at the conclusion of the summer, 
from the dryness of the soil, and the evaporation 
ed by the excessive heat, they are scantily supplied 
sap. In temperate climes their fall takes place in 
when the weather is becoming colder. Their fall is, 
course, always preceded by the cessation of the circulation 
them. This may be partly the effect of their decay 
exhaustion, but it is most probably the principal cause 
that decay. Leaves with the petioles jointed on the s 
fall first ; next, those of which the petioles are not jointed; 
_ lastly, those which are sessile. All leaves fall, sooner or 
_ 216, Plants which are destitute of leaves are called 
le: Glass-wort (Salicornia), Dodder (Cuseuta), and 
_ Of the flowerless tribe of plants, as Lichens, Fungi. 2 
217. There are two sets of veins or vessels in the 
one one proceeding to the upper surface from the ste 
conveying from it the sap for the purpose ‘ 
eee of ee = ‘This surface 
