Cer at op ter is thalictroides , (L.) 109 
Salvinia and Azolla , but in these forms the apical cell, 
which is found at the summit of the cone, is two-sided and 
not three-sided as in the case of Ceratopteris. The cells of 
which the young leaves are composed are arranged in a 
markedly regular manner when seen in longitudinal sections ; 
towards the apex they are in horizontal layers, a single cell 
occupying each layer. Lower down and farther away from 
the leaf-apex vertical walls make their appearance in the 
individual cells, but for a considerable length of time the 
arrangement remains very regular. Near the apex the vas- 
cular strands of the stem are undifferentiated into their dif- 
ferent tissues, but consist of masses of parenchymatous cells. 
The tissue is however readily distinguished from the ordi- 
nary parenchyma by the closeness with which the cells are 
arranged and by the conspicuous and deeply staining nuclei. 
The ramenta are made up of cells which, when seen in 
section, have a shrunken appearance: they are generally 
destitute of nuclei and cell-contents. 
Fig. 10 shows a transverse section through the apical 
region of a stem of Ceratopteris . In this it is seen that the 
cone-shaped apex ( st ) is circular in outline, lying amongst 
the young leaves, which are cut across in different planes. 
C. The Hoot. 
According to Kny 1 , the secondary roots, which are developed 
from the bases of the petioles, are exogenous in origin. 
A root arises, according to his investigations, from a single 
mother-cell lying immediately below the epidermis. Van 
Tieghem and Douliot 2 , however, maintain that this is not the 
case, for each root arises from a single mother-cell which 
originates within the endodermis itself. The roots are 
therefore endogenous and not exogenous, and in this respect 
Ceratopteris resembles the rest of the Filicineae. The mature 
root is traversed by large air-spaces, generally six in number, 
which are separated from each other by strands of tissue or 
1 Kny, 1875, p. 48. 
a Van Tieghem and Douliot, 1888, p. 540. 
