56 
- A. G. Tansley. 
it as a very slight departure from our hypothetical primitive type. 
The tangential extension of the leaf-trace may be regarded as a 
direct consequence of the branching of the frond in that plane, 
such as we find for instance in the bases of the flabellate laminae 
of the erect species of Pallavicinia and allied genera, while the great 
increase in diameter of the stele of the stem is a response to the 
crowding of leaf-traces and the consequent necessity for the supply 
of a great number in a short distance. 
Fig. 3. Tubicanlis Solenites. Showing a type of organisation like that 
of Grammatoptcris but with C-shaped petiolar vascular strands, the concavity 
of the C facing away from the stem axis. After Stenzel. 
The vascular cylinder of the stem of Botryopteris likewise has 
solid xylem, but this contains fairly constantly a central strand of 
small tracheids (Fig. 4) which very likely represent protoxylem, 
Fig. 4. “ Rachiopteris cylindrical' The solid xylem of the stele is pre¬ 
paring to branch, and a second group of small tracheids is seen on the left. 
The branch of the stele may be destined for a stem branch or for a leaf- 
trace. The tissue surrounding the xylem has been put in at the top of the 
figure only ; the large thin-walled elements are probably sieve-tubes. Univ. 
Coll. Collection, K 14. X 50. 
