PHYSIOLOGY. 99/7 
with bones, has certainly not the least likeness to 
them. 
Plants consist of an external or outermost cuticle; 
(epidermis), which, as in animals, is thin and without 
vessels. Below this lies the skin, (cutis), which is 
full of vessels, and which in woody plants is con- 
verted into bark, (cortex). It covers the inner bark, 
(liber), which is solely composed of vessels. This 
is followed by the a/burnum, or the soft wood, as it 
is called. ‘The wood, (/ignum), is inclosed by the 
last, and surrounds the pith, (medulla). 
The inner bark, alburnum, and wood, are one 
and the same substance at different periods of exist- 
ence. ‘Uhe inner bark 1s converted into alburnum,; 
and this into wood. ‘They are all three compressed 
vessels, which are more or less hard, or still soft. 
The pith almost entirely disappears in very thick 
large trunks, by the increasing solidity of the wood, 
and in few plants only remains always throughout 
all parts of the trunk. We find it in herbaceous 
plants, but most aquatic plants want it entirely. 
The stems of herbaceous plants have neither al- 
burnum nor wood. ‘The epidermis surrounds their 
vascular membrane, which rarely in them is convert- 
ed into bark, and in its centre lies a ring of vessels, 
corresponding with what in woody plants is called . 
the inner bark. Immediately beneath this we have 
a more or less dense cellular membrane, (tela cel- 
lulosa), which is often very succulent, and next to 
it, a fleshy substance, (parenchyma). ‘This incloses 
the pith, which in fact is a cellular texture of a dif- 
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