40 Bower.—Studies in the Phytogeny of the Filicales . VIP 
solenosteles are complete, while the latter is beginning to show the incurv¬ 
ing to form the leaf-gap for the departure of leaf bt 
It is thus apparent that the vascular system of a large stem may form 
four concentric tracts ; but that the direct vascular connexions of the leaf 
are with the outermost ring only; while secondary connexions with the 
inner solenosteles copy with less regularity the behaviour of the outermost. 
Fig. 28. a-d. Vascular system of an old plant of Pteris podophylla seen in transverse sections 
successively from below upwards. Natural size. 
The irregularity is greatest in the innermost tracts of vascular tissue. 
Compared with other elaborate vascular arrangements the general dis- 
1 It is worthy of note that an isolated vascular strand makes its appearance in the dilated base 
of the leaf b. It is shown in the sections B, c, D, but it is occasional, and appears only in large 
leaves. It is entirely disconnected with any other vascular tract, upwards or downwards. In fact it 
is a detached island of vascular tissue. Yet it consists of a xylem-core, an outer band of phloem and 
conjunctive parenchyma, and an endodermis. This is a good example of the formation of tissues, 
and especially of endodermis, de novo , and it is worthy of the attention of those who attach impor¬ 
tance to the continuity of such tissues in morphological argument. 
