SPHENOPHYLLEAE. 349 



been observed. We can scarcely hope to give a decisive answer in the case 

 of fossil remains to a question which is sometimes so difficult in dealing 

 with living material. These elements are arranged on the transverse section 

 of the stem in regular radial rows, and at the same time in concentric 

 successive layers ; they touch one another in the four chief planes which are 

 their lateral boundaries. But the truncation of their angles causes the 

 formation between them of irregularly four-angled spaces, which, as the 

 radial angles suffer a similar truncation, communicate with one another in 

 this direction and form a more or less regularly arranged system. This 

 system is seen, when the preservation is good, to be filled with small-celled 

 parenchyma, the elements of which are radial in radial sections, in others 

 vertical and somewhat elongated 1 . This tissue differs essentially from the 

 medullary rays of other plants in arrangement and distribution, but may be 

 its equivalent as regards function. 



The bays in the originally three-winged woody body are soon filled in 

 by a large development of secondary tissue. But this is not brought about 

 in the present case by the introduction of a larger number of tracheides ; 

 the concentric layers are always and everywhere maintained with unvary- 

 ing regularity (Fig. 48, 2,, 4). Only the breadth of the elements changes, 

 and is much greater in the portions of the secondary growth in front of the 

 bays than elsewhere. It is peculiar that there is no gradual passage from 

 the one to the other, but that the small-celled portions in front of the 

 angles, which increase in breadth towards the outside and have parabolic 

 inwardly convex bounding lines, are in quite sharp contrast with the others. 

 In this way the primary strand is surrounded by a closed mantle of a 

 secondary formation of tracheides, which proceed from an external cambial 

 zone and pass layer by layer into the permanent state. Specimens are 

 often found, in which one layer only (Fig. 48, 3) or a few are developed 2 , 

 and these are then always equally and fully developed all round. In older 

 stems on the contrary layers are often observed which have not reached 

 their complete development in every part, and here it looks as if this de- 

 velopment began in the area of the small-celled corner-portions. The 

 growth in thickness may not last very long. In the transverse sections 

 figured by Renault I find the largest number of concentric layers of wood 

 to be eleven, and he says 3 that one stem which he has not figured has 

 fifteen. Williamson 4 indeed has figured a transverse section with a great 

 many more layers, but with Renault I question most decidedly whether this 

 preparation belongs to the group which we are considering. Its secondary 

 wood of scalariform tracheides shows quite normal medullary rays of a 

 single row of cells, such as do not occur in Sphenophyllum. We may 



1 Renault (2), vol. iv, Introd. t. C, ff. 3, 4. a Renault (20), t. 7, ff. 2, 3, and t. 9, f. 4. 



3 Renault (2), vol. iv, Introd. p. n. * Williamson (1), v, t. 4, f. 21. 



