5°6 
Boodle. — On the Occurrence of 
growth being obliquely, ascending. In the later growth of these branches 
the tips become vertical, and on leaving the soil they continue their growth 
as aerial shoots. 
In branches of type (2) Bertrand (’ 81 , p. 265) regards each lateral 
apex as being an arrested and laterally displaced branch of a dichotomy, 
the other branch having continued its growth and appearing like the direct 
continuation of the parent branch 1 . The growth is therefore described by 
Bertrand as sympodial. He also (’ 81 , p. 374, &c.) regards some parts of 
the plant, e. g. certain aerial shoots and branches of category (3), as being 
‘cladodes,’ produced by a kind of cohesion or fasciation of ordinary 
branches, founding this view partly on the structure of the stele. These 
views are referred to by Solms-Laubach (’ 84 , p. 157) and need not be 
entered into further here. 
For the purpose of the present description it will be sufficient to distinguish 
between the different parts as follows : (a) aerial stem ; {b) ordinary rhizome, 
including branches of types (1) and (2) ; (c) type (3) described above, having 
characters transitional between rhizome and aerial stem ; this may be called 
the stock of the aerial stem. The nature of the transition makes it im- 
possible in some cases to draw any sharp limit between the stock of the 
aerial stem and the ordinary rhizome, or even to classify some of the older 
branches 2 . 
In Bertrand’s work the results are given of an elaborate investigation 
of the structure of Psilotum. The figures illustrating this work may be 
referred to for the primary structure of the stele. Thus Fig. 134 (p. 298) 
shows the stele of some very delicate branches of the rhizome, having only 
a small number of tracheides in the xylem, namely four, three, two and 
one respectively. 
A transverse section of a branch of the rhizome having only three 
tracheides is shown in Fig. 45 in the text in the present paper. The stele 
of a stouter branch of the rhizome is reproduced in Fig. 1, PL XXXIII ; 
here nine tracheides are seen 3 . In the ordinary rhizome the number of 
tracheides in the stele varies greatly with the diameter of the organ. When 
only a few tracheides are present, there may be no clearly marked proto- 
xylem, but when the tracheides form a larger group the xylem often takes 
the form of a diarch plate 4 . Fig. 4 6 in the text shows an incompletely 
differentiated diarch plate 5 of eleven elements, of which five tracheides only 
are lignified. Here, and in other cases (e. g. at the base of the aerial shoot), 
1 Solms-Laubach (’ 84 , p. 158) however found that true lateral branching occurred in addition to 
this type of growth. 
2 In doubtful cases the stumps of rhizoids, which have fallen off, give some clue by their 
abundance or otherwise. 
3 Seven are distinct, and there are two pale ones on the left. 
4 See Bertrand (’ 81 ), Figs. 135 and 136. 
5 Differentiation appeared to have been arrested. 
