126 Gibson. — Contributions towards a Knowledge of 
Hofmeister. I do not purpose in the present paper to enter 
into the question of the development of the leaf, reserving that 
point for discussion in connexion with the structure of the 
stem-apex on which I am at present engaged. 
In a brief note read before the British Association in 1887, 
McNab 1 points out that in S. densa , Hort., .S. Poulteri , 
Hort., and .S. Krciussiana , A.Br., a triple row of stomata is 
developed along each margin, one row above, one below, 
and one actually on the margin itself. All three species want 
the sclerotic marginal cells so commonly found in that 
situation. 
Haberlandt’s important paper on the chloroplastids 2 may 
next be noted. In this treatise he gives a very full account 
of the structure and development of the chlorophyll-bodies. 
In the present contribution I have omitted any discussion of 
these, simply because I have nothing of any importance to add 
to Haberlandt’s account. 
I may next refer to Dangeard’s account of the structure 
of the leaf 3 . He distinguishes two distinct types of leaf- 
structure : (a) where the upper and lower epidermal layers are 
similar, and (b) where they are dissimilar. Under the first 
type two sub-sections are constituted : (1) leaves in which the 
mesophyll is homogeneous, e. g. 5 . spinosa , &c. ; and (2) leaves 
in which the mesophyll is heterogeneous, e. g. .S. Kraussiana , 
&c. The leaves in the former sub-section have an epidermis 
composed of elongated (in the long axis of the leaf) rect- 
angular cells, while the mesophyll consists of long, branched, 
narrow cells with numerous intercellular spaces ; in the latter 
section the mesophyll is subdivided into two distinct layers, 
an upper palisade-layer composed of conical cells perpendicular 
to the epidermis, and a lower spongy layer. The type of leaf 
with dissimilar epidermal layers Dangeard further describes 
as having homogeneous reticulate mesophyll throughout. 
1 On the Stomata and Ligules of Selaginella. Brit. Assoc. Reports, 1887. 
2 Die Chlorophyllkorper der Selaginellen : Flora, 1888. 
3 Essai sur l’Anatomie des Cryptogames vasculaires : Le Botaniste, 1889, 
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