Scott and Maslen . — The Structure of Trigonocarpus. 109 
running in bundles which interweave with one another almost like basket- 
work. These cells are followed by the layer of radially elongated elements 
or palisade layer which usually constitutes the greater part of the thickness 
of the sclerotesta, and forms its outermost part. The actual proportion in 
thickness of the palisade layer to the rest of the sclerotesta is subject to 
considerable variation in different seeds. Many of the details of the 
structure of the sclerotesta are shown in Williamson’s figures (Williamson, 
1877, PI. 14, Figs. 1 12, &c.). At the principal ridges the structure is 
somewhat different from that described above, as the majority of the cells 
follow a mainly longitudinal course, so that they are cut across in transverse 
sections of the seed. Many slides show dark-coloured elongated cells, 
passing radially through the centre of each principal ridge, and it is along 
these elements that the splitting which is so frequently seen takes place. 
The general plan of structure as given above, appears to have been 
the usual one in the sclerotesta of the Palaeozoic Cycad-like seeds, and 
a similar structure has been described in detail by Prof. Oliver in Stephano - 
spermum akenioides , Brongn., and 5 . caryoides, Oliver 1 . It also appears to 
agree in a general way with the structure of the stony layers of the seed of 
a modern Cycad. Miss Stopes states that : ‘It is very clearly marked in 
many Cycads that the stone has at least two layers, an inner one of mainly 
vertically running stone cells, as in the integument of Lagenostoma oroides , 
and an outer one of mainly horizontally running stone cells.’ 2 This layer is 
surrounded by the usually undifferentiated cells of the outer flesh. In 
some Cycads the structure of the outer layers of the testa is much more 
complex, as in Cycas Beddomii , described by Miss Stopes 3 . In this form 
the stony layer (sclerotesta) and outer flesh exhibit the following details of 
structure. The innermost part of the stone is formed of layers of vertically 
elongated sclerotic cells comparable with the inner zone of Trigonocarpus 
P arkinsoni as described above. These are succeeded on the outer side by 
a number of layers of elongated sclerotic cells which follow a horizontal 
direction and run circularly round the seed. A similar zone is found in 
Trigonocarpus. Next follows a thick layer of radially elongated horizontal 
cells, which are described as belonging to the outer fleshy layer. These 
cells have somewhat thickened slightly lignified walls, and may be com- 
pared in a general way with the outer layer or palisade-layer of the 
sclerotesta of Trigonocarpus. The radially elongated cells pass externally 
into others with reticulately thickened walls, and these again into the 
ordinary tissue of the outer flesh, consisting of large parenchymatous cells 
with pitted cellulose walls similar to the ordinary sarcotestal cells of 
Trigonocarpus. The reticulately thickened tracheide-like cells, which occur 
between the radially elongated inner cells and the ordinary parenchymatous 
1 Oliver (’ 04 ) (i), p. 364 and p. 373. 2 Stopes (’ 05 ), p. 562. 
3 Stopes (’ 04 ), p. 444 and Fig. 7. 
