102 REPORT ON BOTANY, MDCCCXLl : 
exhaled cambium (exhale), with an imperfect organization. 
This gelatinous zone soon organizes itself, and becomes paren- 
chyma, in which new tracheae are situated on the external, 
and new proper vessels on the internal side of the older 
vessels, and between them a new gelatinous intervening space 
is formed. An analogous growth also takes place in the 
medullary rays. Some vessels remain stationary at the first 
period of their growth, their vascular bundles are always round ; 
the gelatinous intervening space of the bundles becomes solid, 
and thus also the analogous intervening space in the pith 
forms rays. In the second period the parenchymatous bundles 
are lengthened through the external part of their central por- 
tion, and through the internal part of their bark portion ; 
these growing portions also spread laterally. By this the 
central portion of the bundle becomes triangular, and strives 
to effect a combination with the adjacent bundle. The cellular 
intervening spaces of the bundles, which are situated between 
the vascular masses, correspond with the intervening spaces 
of the other bundles, and in this way the medullary layers 
(circonferences medullaires) are formed, which, however, do 
not always fit exactly one upon another. The new vascular 
groups do not, however, attach themselves immediately to the 
old, but there is almost always a separating cellular tissue 
between them. This is very easily to be distinguished in the 
roots of the beet. The bundles meet at the end of the first 
year, and thus is produced the first annual ring. New trans- 
parent layers are produced between the wood and bark in the 
third and fourth period of the growth, from an exhaled cam- 
bium ; new vascular fibres are produced in the parts which 
belong to the central system, and are separated by medullary 
layers and medullary rays, which are either continuations of 
the preceding year, or new formed ones. At last, the internal 
layers of wood, and the external layers of bark, become dense 
and hard. The root of the Bicotyledons is similarly formed 
to the stem ; and although the pith does not appear in many 
roots, it yet penetrates into it, gradually decreasing. Of the 
leaves the author says, that the position of the leaves on the 
stem have hitherto alone been investigated, without seeking 
494 
