244 
MORPHOLOGY 
with the insertion of the leaf traces. It will be remembered that the 
presence of leaf gaps is a feature of the ferns, in contrast with the other 
groups of pteridophytes; and their appearance in the dicotyledons is 
taken to be one indication that this group is connected with ferns, either 
through gymnosperms or directly. In tracing the development of 
the vascular system in a seedling dicotyledon, it is interesting to note that 
the stem cylinder 
often begins as a 
protostele, and more 
or less rapidly be- 
comes a siphonostele. 
Monocotyledons. 
The monocotyledons 
were once thought to 
be the primitive 
angiosperms, but the 
study of their vascular 
anatomy has been 
chiefly instrumental 
in suggesting the 
probability that they 
are derived from 
dicotyledons. The 
evidence is obtained 
from a study of the 
development of the 
vascular system from 
the earliest stages of 
the seedling to the 
adult stem. A trans- 
verse section of an 
adult stem usually shows " scattered " vascular bundles (fig. 550), quite 
unlike the arrangement into a hollow vascular cylinder characteristic 
of the dicotyledons. In studying the development of this stem, how- 
ever, four stages are often recognized. In the earliest stage the cylinder 
may be a protostele; and this passes more or less quickly into the second 
stage, that of the siphonostele, in which the cylinder is just that of a 
dicotyledon, with its collateral bundles. This means that an embryonic 
stage of a monocotyledon is the permanent, adult condition of a dicoty- 
FiGS. 547, 548. Tracheids: 547, those of gymnosperms, 
with bordered pits (after CHAMBERLAIN); 548, the scalari- 
form tracheids of ferns (after DEBARY). 
