Scott and Hitt.—Structure of Isoetes Hystrix. 423 
however, distinguish between the latticed cells and the ordinary 
intracambial parenchyma. This distinction, as already men¬ 
tioned, was first drawn by Hegelmaier, but he failed to make 
out the latticed structure of the ‘ empty ’ cells, owing no 
doubt, as he himself suspected, to defective optical appliances 
( 1 . c., p. 503). Farmer (1890, p. 42) confirms Hegelmaier’s 
account of the alternating zones. 
Russow regarded the secondary intracambial tissue as 
phloem, which is no doubt correct, but with the limitation 
that only the latticed elements, forming part of that zone, are 
the true phloem. The parenchyma alternating with the 
phloem-bands is best regarded as secondary ground-tissue, 
not as phloem-parenchyma, for there is no reason to associate 
it with the phloem any more than with the secondary xylem, 
which forms part of the same region. 
Speaking generally, the whole of the phloem in the stem of 
Isoetes must be regarded as secondary, for it belongs to the 
tissues cut off on the inner side of the cambium. It is not 
usually possible to identify any primary phloem with certainty. 
In fact the tangential divisions begin so early that it would 
scarcely be practicable to distinguish between primary and 
secondary structures, at the periphery of the stele. In certain 
cases, however, the cambium at its first origin is a normal one, 
the phloem-elements lying on its outer side, and in these 
instances the extra-cambial phloem may be reasonably re¬ 
garded as primary. (See Fig. 8.) It may here be pointed 
out that the tissue in immediate contact with the primary 
wood, which Hegelmaier inclined to regard as phloem (1874, 
p. 502), is certainly not of that nature, as it is always parenchy¬ 
matous, with none of the characters of the phloem. 
Not infrequently the cambium, at its first origin, cuts off 
a few secondary xylem-elements in contact with the primary 
wood (Fig. 7, x 2 ). In one stem the only secondary wood found 
was in this position. These indications of a normal cambial 
development are of some interest, as suggesting the possi¬ 
bility that the anomalous secondary growth of Isoetes may 
have been derived from a more regular mode of thickening, 
