80 
REVIEW. 
the property of perpetual reproduction and extension, a question 
follows :—How is this subdivision effected ? In the case of bulbs it 
has already been stated, that the radicle plate is composed of an end¬ 
less train of gems, which are developed in the order of their senio¬ 
rity ; tubers are multiplied by divisions or branches ; fibrous-rooted 
herbaceous plants perpetuate themselves by lateral offsets; hut how 
is the annual subdivision of the vital envelope of trees accomplishedP 
To this question a direct answer cannot be given, because the process 
is invisible; but we can gain a knowledge of the changes which take 
place between the wood and the liber of a tree, by making incisions 
through the hark, and marking the changes during the spring, sum¬ 
mer, and autumn growth. 
8 
a b c d e 
Transverse and perpendicular sections of a stem four years old ; the latter 
through the pith, a, Pith and wood of the first year; b, c, d, layers of wood of 
the seoond, third, and fourth years; e, the four thin layers of bark. 
About the end of May, sooner or later, according to the favoura¬ 
bleness of the season, similar sections of a stem of the same age will 
appear as Figure 9. 
9 
Sections of a stem as it appears in May or June of the fifth year. The white 
places showing the swelling cambium. 
