46 ELEMENTS OF PLANT ANATOMY. 
in length, are not strikingly different. In this respect they form 
a single tissue, called parenchyma, a name applied to cells nearly 
or quite isodiametric. 
The next change is one which gives rise to prosenchyma, or 
cells with one long axis, and the process may be described as 
follows. A cross-section made just below the stem apex shows 
clusters of cells differing from the others by their shorter radial 
and tangential diameters. A long section through one of these 
clusters shows that these 
cells have one long diam- 
eter corresponding with 
the long axis of the stem. 
They arise by certain 
meristematic cells ceas- 
ing to divide transversely 
and growing faster in the 
longitudinal direction,un- 
til that diameter is several 
Say ae times as long as the cor 
Cross-section through stem tip of Heterocentron pea aneniere aad 8 me Rs 
diversifolium with two cambium bundles. — responding diameters of 
Accd. to Véchting.) : 
( eee hie surrounding cells. 
These are the cambium cells, and from them are derived the 
vascular bundles. We have now the origin of the two principal 
kinds of tissues as regards the shape of the cells. The cambium 
cells, after reaching the necessary length, resume their meri- 
stematic nature and divide rapidly by forming new walls parallel 
with their long axis of growth ; therefore cells originating from 
the cambium are from the first long and narrow. Owing to this 
character they are called prosenchymatic or prosenchyma, while 
the cells derived from the other meristems are parenchymatic. 
It is not true, however, that all cells derived from the cam- 
bium tissue remain prosenchymatic, as in certain instances such 
cells form transverse walls and are thus divided into a number 
of parenchymatic cells. On the other hand, there are instances 
