Scott and Hill.—Structure of Isoetes Hystrix. 441 
taneous over its whole area, but in some cases a centripetal 
development can be traced. 
4. The cell-division of the primary meristem passes over 
without any interruption into that of the cambium. 
5. In some cases the cambium is at first normal, lying on 
the inner side of the first-formed elements of the phloem, and 
producing secondary tracheides in contact with the primary 
wood. In this case a second cambium soon arises further to 
the exterior. In other specimens the more internal cambium 
arises after the other. 
6. As a rule, the same cambium is active throughout, 
producing secondary ground-tissue, wood, and phloem, on 
its inner side, and cortical parenchyma only, towards the 
exterior. 
7. Secondary wood is always formed, though very variable 
in amount. Its elements are typical tracheides, without cell- 
contents. 
8. Well-differentiated phloem, consisting of enucleate ele¬ 
ments, with clathrate pitting on their walls, is always present 
in the intracambial zone, forming definite, concentric bands, 
alternating with the secondary parenchyma. The phloem of 
the stem is continuous with that of the leaf-traces. 
9. The stele, cylindrical above, becomes triquetrous below, 
owing to the abutment upon it of the root-bases, successively 
developed along lines corresponding to the furrows. 
10. The development of the roots in each series is acropetal, 
while that of the several series belonging to each furrow is 
centripetal, with reference to the centre-line of the furrow. 
11. The downward growth of the base of the stem is 
entirely due to the activity of the cambium, and to the 
addition of new root-bases. 
II. The Leaf. 
The general structure of the leaf is too well known to need 
recapitulation. 
1. The vascular bundle, which is collateral throughout, has 
exarch structure in the lamina, the protoxylem lying next the 
