Farmer —On Isoetes locus tris , L. 
4i 
an upward and outward direction, which more or less irregu- 
larly connect the base of the leaf-rudiment with the central 
part of the stem, at the apex of the woody portion of the 
bundle. Thence the division extends upwards into the leaf 
and downwards into the stem. In all cases where I have 
been able to observe accurately, the tracheids of the trace 
are seen to join the axile strand in such a way as to afford 
conclusive evidence that they, with the tracheids of the 
neighbouring traces, form the entire mass of wood. No 
xylem-elements, so far as I could determine, are ever present 
above the insertion of the last leaf-trace. 
The elements of the wood in the vascular cylinder of the 
stem are very short as compared with the corresponding cells 
of the trace, and are largely mixed with conjunctive paren- 
chyma. At a short distance below its summit, the axile 
cylinder of vascular tissue in the stem shows signs of irregular 
splitting, owing to the disruption of its cells, and the whole 
structure rapidly becomes full of intercellular spaces. This 
result is indirectly brought about by the activity of the cam- 
bium. The traces of the roots and leaves which pass through 
this zone to join the axile vascular cylinder are subjected to 
great strain by the fact that whilst the cells round them are 
growing, and, so to speak, travelling outwards, they themselves 
are stationary, and can only increase in length by extension. 
This takes place to a limited extent, and effects a great dis- 
tortion of their constituent cells, but finally the loose central 
tissue gives way, and they are partially relieved of the strain 
at the expense of the cohesion of the inner tissue. That such 
really represents the actual state of the case may be ascer- 
tained by inspecting longitudinal sections through parts of the 
stem where the process is being vigorously carried on, and it 
is indicated by the character of the cells in the photograph 
(PI. VI. Fig. 27). It is difficult to assign any other reason for 
the occurrence, mere extension of the cells of the stem will at 
any rate give no explanation. 
The cambium formed as above described, besides giving off 
cells peripherally, produces also the so-called prismatic cells 
