[kirscH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 401 
cavities of the intercellular canals of the bundles, and blocked them more 
or less completely. The air passages which occur in the fundamental 
tissue of the petioles of this form were also blocked up by the thylosal 
growths. 
From the above it is evident that the well-known intercellular canals 
in the vascular bundles of the Mono-, and Dicotyledons and of the Equi- 
setaceæ, present parallel stages of development with those observed in the 
various orders of the Filices and in the Ophioglosseæ, and that they are 
all identical in every respect. Having accomplished this, it will now be 
well to return to Pteris aquilina, and, if possible, determine the probable 
function of the canal by means of the data obtained during the investi- 
gation. Since, however, the canals observed in the other forms are 
homologous with those in Pteris, what is found to be true of the latter 
will also hold good for them. 
Before proceeding to deal with the function of the c:nal in Pteris, 
it will be instructive to note what function is attributed to the homo- 
logous structure in Equisetum. 
Campbell, in describing Equisitum telmateia, states the following 
(2, 464) :—%A cross section of the fully-developed stem of E. telmateia 
(Fig. 266, C) shows this very regular arrangement of the vascular 
bundles and lacunæ. In addition to the large cortical ones, each vascular 
bundle has on the inner side, a large air space, which like the other is 
formed by the tearing apart of the tissues of the bundle.” He thus con- 
siders the carinal and vallecular canals to perform the same function, 
viz.: that of an aiz-passage. 
Strasburger, on the other hand, states that the function of the 
carinal canal differs from that of the vallecular canal, as can be seen 
from the following (20, ) :—“ Each axis (of Equisetum arvense) 
consists of a series of elongated internodes...... internally it is tra- 
versed by a central air-passage and by a number of smaller peripheral 
passages, vallecular canals...... Between the central and the vallecular 
canals and alternating with the latter, is a circle of bicollateral vascular 
bundles...... Each vascular bundle is also traversed by a longitudinal 
water-passage, the carinal canal.” Strasburger bases his view on experi- 
mental data (19, 437), and so it will be taken for granted that the func- 
tion of the carinal canal is the conduction of water. 
It has been shown in Pteris aquilina and in some of the other 
Pteridophyta that the canal always arises at a very early stage in the 
growth of the plant, and before the secondary xylem has developed to 
any extent. It is the expansion of the growing secondary xylem elements 
which causes the separation of the already formed protoxylem elements, 
Sec. IV., 1907. 25. 
