16 Ontogeny 
young plants of the more bulky forms like Angiopteris and 
Marattia have more .or less elongated stems, so that a primitive 
type with an elongated axis is distinctly suggested. Such a 
primitive type may have been solenostelic. On the other hand it 
maj' have had a tubular stele without internal phloem, as in 
the Ophioglossacea:, and the phloem and endodermis subsequently 
developed on the internal side of the separate strands. With the 
evidence at present available it is very difficult to decide between 
these alternatives. 
In the young plants of Ophioglossaceae, when the cylinder has 
passed out of the protostelic condition, a pith appears and a distinct 
internal endodermis is developed. This disappears in the older 
plants of Botrychium and Ophioglossum, but remains in Helmintho- 
stnchys, though in a degenerate condition. There is no evidence 
from the young, plants that internal phloem ever existed in the 
group. 
