7^ ANALYSIS OF THE SHOOT-SYSTEM OF FERNS [CH. 



Once the inequality of development of the shanks of dichotomy is 

 established in any Fern, a sympodial succession of them becomes possible, 

 so as to form what has been aptly described by Velenovsky as a dichopodiuvi, 

 that is, a false axis made up of a succession of bases of dichotomy, such as 

 is shown diagrammatically in Fig. 70, j5. In these cases, which are common 

 in Ferns, the more the weaker shanks are side-tracked the more the)' appear 

 as mere appendages upon the dichopodium, till the final interpretation of the 

 branching that gave rise to them may become a difficult problem. This can 

 only be solved on the basis of development, anatomy, and comparison studied 

 in each several case. Moreover, a further difficulty in the solution arises from 

 the general fact that where a part is partially arrested in its development it 

 is apt to be delayed in its time of appearance. This leads to the result that 

 the branching becomes technically monopodial, though still it may be held 

 to have been dichotomous in origin. Notwithstanding such difficulties the 

 branchings of Ferns may be very generally interpreted as derivative from 

 distal dichotomy. 



Occasionally more than a single bud may be found at the base of a 

 leaf This occurs in Pteridiiim (Fig. 70, A\ and it has been noted by 

 Gwynne-Vaughan for Hypolepis repens. In the former case an explanation 

 based on a second dichotomy has been suggested by Velenovsky, and it is 

 indicated diagrammatically by Fig. 70, B. In Hypolepis a similar explanation 

 is also possible. Gwynne-Vaughan notes for Hypolepis repens that if one bud 

 only is present its vascular supply always arises from the basiscopic margin 

 of the leaf-trace: if there are two or more, then the lowest on the basiscopic 

 side is always stronger and further developed than the others. This appears 

 to be so also in Pteridiiim, and the regularity thus observed strengthens the 

 opinion that the buds are referable to a regular branching rather than to 

 adventitious development. But developmental observations will be necessary 

 before a final opinion can be formed on these difficult cases (Fig. 71). 



On the basis of dichotomous branching of the axis, carried out inde- 

 pendently of any one leaf either as regards orientation or identity, and with 

 unequal development of the two shanks of the fork, it is possible to account 

 for certain other well-known branchings in Ferns. If an unequal dichotomy 

 be associated with the regular formation of a leaf on the free side of the 

 weaker shank, the result would be an axillary bii-d (II). This condition is 

 seen in the Hymenophyllaceae and Ophioglossaceae, among living Ferns. 

 An early condition of the bud is shown in Fig. 63, together with its vascular 

 connections, which indicates that the axillary bud and the subtending leaf 

 have a common vascular supply, a relation which agrees with the theory of 

 dichotomy. In that case the subtending leaf would be the first borne upon 

 the arrested shoot lying in its axil (compare Chambers (60), p. 1037). The 

 fact of axillary branchings is also well authenticated in Zygopteris and 



