52 THE STEM. 
° 
165. In herbs, and in some trees, the pith continues of the — 
same diameter during the whole life of the plant. In many 
vegetables the pith disappears as the plant grows, and the 
stem becomes at last perfectly hollow, often, however, lined 
with a thin coating of pith resembling cotton. This is the 
case in many of the Umbellifere, as in Hemlock. 
166. The pith, it is supposed, nourishes the young wood 
and the buds during the first year of their existence; and 
it has been observed that it retains its moisture for a lo 
period near the terminal bud, and at the parts where b 
ches are given off. The medullary sheath is continued into 
the buds and leaves, forming part of the veins of the leaves 5 
and is believed to contain air with a large proportion of oxygen. 
167. Such is the structure of the stems of that very large 
class of plants which constitutes the third division. ‘They 
are found only in Dicotyledonous plants, and they are called 
Exogenous, because the wood, which is the principal part: 
them, increases in diameter by the addition of new matter 
its external surface. a 
168. In exogenous plants, the new matter being added 
externally, a bark or covering is necessary to protect it, w 
_ young and tender, from the action of the atmosphere, aul 
_ from external injury from other causes: hence an i r 
ant office of the bark. In endogenous plants, the new mat 
ter, being added internally, is provided with an excellent 
covering, formed of the main substance of the plant, and 
has no need of a separate protecting integument. et 
169. In spring there is found between the bark and the 
alburnum, a viscid gelatinous fuid, called cambium, whic 
_ it is supposed, is a principal agent in forming the new k 
ers of wood and of bark. This fluid is 
