The Cell as a Member of a Group. ‘ae 
period of their existence, chiefly or even exclusively from 
the formation of a special generating tissue, the cambium. 
~The cambium 1s usually a prosenchymatous tissue, not found 
in the ‘ punctum vegetationis,’ but distributed elsewhere in 
very various parts of the plant. To it is due the final deve- 
lopment of the fibrovascular bundles, which however originate ~ 
in its forerunner, the procambium. The cambium forms, 
according to its arrangement, a central cambium-cylinder, 
or isolated dispersed cambium-bundles, or a cambium- — 
ring. A thickening-ring of this character occurs, for 
example, in the layer formed between the wood and the bark | 
of those trees which are characterised by the formation of 
annual rings. In the most lowly organised plants, the Fungi, 
Lichens, and Algz, both kinds of generating tissue are 
wanting, and every one of the cells has an equal capacity for — 
originating new vegetative cells, ze such as serve only for | 
growth and not for reproduction. Although the production 
of new cells is the special province of the specific generating © 
tissues now described, this does not imply that the elements 
of other tissues are entirely destitute of this power; on the 
contrary, it is strikingly manifested in the annual and espe- _ 
cialiy in succulent parts of plants. 
In direct contrast to the generating tissues are the — 
_ healing-tissues, suberous tissues, or cork-tissues. The former 
are of service to the plant by their long-continued and vigor- 
ous growth and the formation of new cells ; while the latter, 
dying quickly, lose their cell-sap ; their cell-walls become con- 
verted into cork, and thus form a mantle or envelope which 
protects the subjacent cells from injurious influences of all 
kinds. Two kinds of this tissue are distinguished—true cork © 
or suber, and periderm; the two are not, however, sharply 
separated, but pass into one another in a variety of ways. 
‘True core is a very elastic tissue consisting of thin-walled, 
usually nearly cubical, cells (Fig. 66). It does not peel off, 
but often contains long clefts, as is commonly seen in the 
bark of the maple. 7 
