592 Ford. — The Anatomy of P silo turn triquetrum. 
In the case of the aerial stem the branching, as a rule, is fairly regular, 
and it might for some purposes be convenient to term the series of bifurca- 
tions as branches of the first, second, third, &c. order. Taking Professor 
Bertrand’s sub-divisions as a whole, there is nevertheless some doubt 
whether such detailed and often confusing distinctions are satisfactory. 
In regard, however, to the rhizome of Psilohim , it must be stated that the 
above sub-divisions 1 have not been confirmed, although it may possibly be 
due to less well-developed material. The branching in the rhizome is 
as a rule very irregular, although here and there dichotomy may be found. 
In other cases, judging by the external appearance, ordinary lateral branches 
are found ; these are generally smaller than the main branch from which 
they are given off, or again they may remain as dormant lateral buds, 
which may develop under suitable conditions. 
Seeing that the apices of the branches are naked, they are in consequence 
easily injured. When this occurs, microscopical, as well as superficial, 
investigation shows that the apex itself is black and discoloured, whilst one 
of the lateral buds lying in its neighbourhood frequently exhibits distinct 
signs of renewed activity. 
The question as to the presence of Professor Bertrand’s centres of 
growth, as shown by the existence of the apical cells and the distinct proto- 
xylem-groups, will be referred to later in dealing with the anatomy of 
the plant. 
Anatomy. 
The Aerial Stem. 
Psilotum possesses a single central stele (PL XXXIX, Fig. i). The 
stem is ribbed in the stouter parts of the plant ; towards the apex it is 
triangular in outline. The epidermis is cuticularised, and stomata are 
present in fairly large numbers lying in the grooves of the stem between the 
ridges (Fig. i, st). The guard-cells are slightly sunk below the level of the 
other epidermal cells, and the outer walls are cuticularised. The cortex 
is composed of three zones, an outer, which is parenchymatous and assimi- 
lating, a middle sclerenchymatous, and an inner parenchymatous surround- 
ing the stele. The outer parenchymatous zone (Fig. i , pa) is composed of 
peculiar cells, the structure of which is best seen in longitudinal sections 
(Fig. 3). The lateral walls bulge out at regular intervals, each cell having 
2-4 such swellings. The swellings of adjacent cells touch each other, 
and in consequence a conspicuous row of intercellular spaces is formed. 
The cells themselves are nucleated and contain chlorophyll. The scleren- 
chymatous zone (Fig. 1, sd) consists of thick-walled fibrous elements with 
pointed ends, and the walls are perforated by small slit-like pits. Towards 
the centre this sclerenchymatous zone passes gradually into the inner 
1 Solms-Laubach (’ 84 ), p. 158. 
