if 
Bllecu Thee ;— Exogen 
Agave AAiEiiiaiNA ; — Endogen. 
I 
ON THE AGE OF TREES. 
N speaking of the age of trees, -we insensibly use the term age, in the same manner as we do when 
speaking of animals. We talk of old trees, old animals, and old houses, as if the same processes had 
gone on in one as in the other. Yet, when we come to examine the natm-e of the changes which have 
taken place during the age of the one and the other, how different they are. Not more does the pro- 
cess of change in the old house differ from the tree and animal, than does the same process in these two. 
The animal has but one life, and this life is dependent on the harmony of the whole organization ; 
whilst, if we examine a tree, we shall find it has (so to speak), many lives, Each bud is capable of an 
independent existence ; nay more, many parts of the tissues of plants are capable of producing buds, and 
each cell has its separate and independent existence. It is here, then, that we see how different must 
be the cii'cumstauces under which age is attained in a tree, from those wliich produce it in an animal. 
Plants are called annual, biennial, or perennial, as they endure for one, two, or more years. The 
difference depends on this, that the tissues of some plants are miable to resist the meteorological influ- 
ences to which they are exposed so well as others. The reason of this difference in the tissues is not 
well made out. It is, however, well known, that a plant in tliis country may be an annual, on the 
Continent, a biennal, and, in the Trollies, a slu'ub, or tree. This is the case with the castor-oil 
plant, (^JRicimis communis.) 
In perennial plants, the tissues which resist ehmatal change cari-y on a kind of low vitality, 
as seen in the trimks of ti'ees, in this country, in the winter. At more favoui'able periods, these 
tissues begin to grow in certain directions ; buds, and leaves, and new tissaes are formed, and depo- 
sited, in vai'ious parts of the plant, more especially covering the old, and growth or increase is 
the consequence. Even in plants not producing leaves, this process goes on, and, year after yeai', new 
tissues ai-e added to the old. This is especially evident in sea-weeds, which thus exist thi-ough very 
long periods of time. Thus, Professor Schleiden says, " On the great fucus bank of Corvo and Flores, 
we might yet find, floating about, plants of Sarffassiim, which had been cut in strips by the 
bark of Columbus ; and, in the northern drift, we might expect to discover Lichens, that had been 
(n 
