Development in Equisetum . 79 
Further, in Equisetum maximum the absorption of the pith is 
absent in the rhizome, partial in sterile portions of the aerial 
branches, and complete in the sporangiferous portions. Thus 
absorption of the diaphragms is not always found in Calamitae, 
and is not unknown in Equisetaceae. 
Neither can great importance be attached to the fact that 
the leaves of Calamitae are free, while those of Equisetaceae 
are united into a sheath. The leaf is known to be the most 
plastic of organs, varying in form in even closely allied species ; 
and the amount of cohesion of foliar organs may vary in 
a single plant, as is the case with the stipules of certain 
Stellatae, where the stipules of opposite leaves may remain 
perfectly free, or be free only towards their tips, or cohere 
entirely. Thus no great systematic importance could, under 
any circumstances, be attached to the cohesion of the leaves 
of Equisetaceae ; still less, when the cohesion may be correlated 
with the weak development of the wood of Equisetum as 
compared with that of Calamitae. The leaf-sheath, by its 
peculiar structure 1 , is fitted to give some slight support to the 
stem of an Equisetum which would be superfluous to the 
woody stem of a Calamite ; and expanse of leaf is less 
important in the case of a distinctly herbaceous stem. 
The foregoing conclusions convince us that Williamson was 
more than justified in maintaining, as he so long has done, 
that the presence of secondary thickening is no sufficient 
reason for removing any of the Calamitae from among the 
Vascular Cryptogams ; and lead us to think that these plants 
resembled Equisetaceae in their vegetative organs even more 
than Williamson admits, inasmuch as certain distinctions 
which he has drawn disappear on closer investigation. 
Renault, in maintaining his contention that these plants are 
gymnospermous, assigns to them certain sporangiferous spikes 
whose spores he calls pollen-grains. But in the absence of 
any proof that such spores produced pollen-tubes, it would be 
unwarrantable to apply to them any more definite term than 
1 Muller, Ueber den Bau der Commissuren der Equisetenscheiden, Jahrb. fur 
wiss. Bot., 1888. 
