Hymenophyllaceae , Schizaeaceae and Gleicheniaceae . 531 
able strains unless the petiole possessed almost complete 
rigidity, and the latter would be unsuitable for positions 
exposed to wind. 
Thus the theory suggested with regard to the origin of 
a bundle-system like that found in the petiole of Pteris 
aquilina , is that a band-shaped primitive bundle became 
arched and afterwards convoluted and divided 1 . 
We therefore arrive at a different view as to the origin 
of the vascular bundles in the petiole of Pteris aquilina from 
that which the writer would suggest for the vascular system 
of the stem of the same species 2 (see Jeffrey, ’00, pp. 10-11). 
Jeffrey in his recent paper (’02, p. 143) states that it is not 
easy to see why on the views put forward (Boodle, ’ 01 ) ‘ the 
same view [that applied to the bundles in the adult stem of 
Pteris aquilina\ should not be taken of the equally complex 
arrangement of bundles in the petiole.’ What has been said 
above explains the writer’s view 3 . It should be pointed out 
that, when one is dealing with a question of morphology and 
comparing the tissues in two different organs, it is necessary 
to form a definite theory as to the phylogenetic history of the 
tissues in both organs before formulating their morphological 
relations. 
Further Anatomical Details. 
The resistance to strong sulphuric acid shown by cell-walls 
in the cortex, pith, &c., of Schizaea digitata has already been 
1 A petiolar bundle of the horseshoe-type may also become closed (presumably 
by the conversion of the tissue between its ends into vascular tissue), e. g. in some 
species of Gleichenia (Boodle, ’01 a, PI. XXXIX, Fig. 19 ). In this case the cen- 
tral tissue of the bundle is regarded as belonging to the historically non-vascular 
portion of the petiole, which has been invaginated ; so the view corresponds 
to that held by Jeffrey for the solenosteles in stems. 
2 For the stem the theory suggested is that if one could follow the stages in the 
evolution of its structure, one would find a protostele converted into a solenostele 
by the replacement of its central tissue by parenchyma and phloem (the two 
tissues appearing successively or simultaneously), and then by the conversion 
of part of the central parenchyma into vascular strands. 
The non-stelar nature of the petiolar bundle is not insisted on here, but simply 
the theoretical view that the centrally placed parenchyma in the petiole has 
not been directly derived from vascular tissue. 
O O 
