512 Boodle . — On the Occurrence of 
have been reduced to scales, whereby reduction of transpiration must have 
been achieved. 
The mode of occurrence of the secondary tracheides appears to point 
to the plant having retained the power of forming such additional elements 
in consequence of the necessity for an adequate water-conduction or water- 
storage capacity in branches, on which the water-supply of well-developed 
aerial shoots devolves. The stimulus, which leads to the differentiation of 
the secondary tracheides, may perhaps be either some secondary effect of 
increased transference of water caused by the transpiration of the aerial 
shoots, or the backward conduction of carbohydrates from the latter. 
The specimen shown in Fig. 44 in the text presents some instructive 
features in relation to this. Secondary tracheides are present in the base 
of the aerial shoot g, throughout the branch (d, c ) bearing it (though de- 
creasing between d and the insertion of /), and downwards in the parent 
stem to the base of b, a. This may well be due to a backward stimulus from 
the shoots f and g. The other aerial shoot (e) had evidently been cut or 
broken off at e when young. In its upper region primary differentiation 
had been arrested before completion, but in its lower region, though primary 
differentiation was complete, there were no secondary tracheides. In 
tracing the structure upwards in the stock of this branch from b towards 
the point where c , d was given off, it was interesting to find the secondary 
tracheides gradually becoming restricted to one side of the stele, so that 
when the branching took place they all accompanied the stele supplying 
d , c. This disposition certainly favours the supposition of a basipetal 
stimulus. 
If one adopts the view that the outer tracheides represent reduced 
secondary xylem, the correspondence between the stem-structure of some 
parts of the plant in Psilotum and that of Sphenophyllum becomes rather 
striking. Setting minor details aside, the triarch xylem-mass surrounded 
by a small amount of secondary xylem in a young stem of Sphenophyllum 
(Scott, ’ 00 , Fig. 35, p. 85, and Williamson, 74 , PI. I, Figs. 2-4) compares 
well with the structure of the stock of an aerial stem of Psilotum seen 
in Fig. 6, PI. XXXIII. The structure of the aerial stem of Psilotum with 
its stellate xylem bears a strong resemblance to that of the axis of 
Cheirostrobus \ but there is the difference that in Cheirostrobus the tracheides 
form a solid star, while in Psilotum the more central part of the xylem has 
been replaced by sclerotic tissue. Comparison of rhizomes cannot well be 
made, as an organ of this kind is not known with certainty in Sphenophyllum , 
and the rhizome of Psilotum may perhaps have been greatly modified by 
1 See Scott, ’97, PI. 1, Phot. 3. Cheirostrobus most resembles Sphenophyllum, though it 
shares certain characters with Lycopods and Calamarians. See Scott, ’97, p. 26, &c. : ‘ We may 
hazard the inference that Cheirostrobus as well as Sphenophyllum sprang from a very old stock, 
which existed prior to the divergence of the Lycopods and Calamarians.’ 
