THE PINNA-TRACE IN THE FERNS. 375 



supply the pinnae, while the abaxial portion of the leaf-trace would cany forward the 

 water-supply up the leaf. There is the possibility in the marginal type of " manu- 

 facturing " so much more tracheidal tissue than is really necessary for the pinna- 

 supply that no contraction would take place between the pinnae. But the actual 

 number of tracheides does not seem to be altogether what keeps the water-supply in 

 the proper direction. There is a deflection of the water-current towards the pinnae. 

 It is really largely this deflection which has to be counterbalanced by the provision 

 of an accessory set of tracheides which will carry the water forward. Where the 

 pinnae are large the deflection will be great, and the counterbalancing hooks will be 

 large ; where the pinnae are small the deflection is often negligible, and the hooks are 

 small. This appears to lead to the conclusion that the Fern-leaf in phylogeny reduced 

 its pinnae, the hooks of the leaf-trace disappearing in relation to this. When a 

 large leaf appeared later, an accessory tracheidal system developed in order to 

 counterbalance the deflections which would have drained the upper part of the leaf 

 of its legitimate water-supply. 



All the features of the various leaf-traces cannot be fully explained on these 

 theoretical grounds, but such distinctive characters of various leaf-traces as the 

 incurved adaxial hooks connected with extramarginal supply to the pinnae, the 

 reinforcement of the adaxial strand in Ferns with marginal supply, and the retention 

 of the marginal type even by fairly large-leaved species can certainly be interpreted 

 if we consider broadly the needs of the pinnae situated towards the tip of the leaf 

 and the demand for water by large pinnae of leaves possessing leaf-traces with 

 unincurved adaxial edges. 



A very interesting confirmation of the view that the adaxial or abaxial hooks of 

 the adaxial portions of the leaf-trace serve to carry forward the water-supply past 

 the pinna-departures is found in Salisbury's paper on " The Determining Factors in 

 Petiolar Structure." He calls attention to Sinnott's conclusion {Annals, xxv. p. 187) 

 that it is " extremely doubtful if the size of the transpiration current has had much 

 influence on the development of the vascular supply," and confirms it for a species of 

 Polypodium. There is a marked contraction of the xylem in a Fern-petiole, as 

 Sinnott reiterates (pp. 187, 188), as we approach near to its point of junction with 

 the stem. Salisbury, in emphasising the restriction of this contraction to a short 

 distance at the base of the petiole, quotes (p. 266) the interesting observations of 

 Ormsby, which show that the effect of contraction in water mains is negligible where 

 the length of contraction is small as compared with the total distance traversed. 

 But there is a possibility of contraction of the water-supply to a great part of the 

 leaf at other points than just at the base. The departure of every pinna might 

 cause such a contraction in the supply for the rest of the leaf. Only in a leaf of 

 considerable length with a single large pinna (if we can imagine such a leaf) would 

 it be possible to neglect the effect of the departure of the pinna-trace upon the 

 supply for the rest of the leaf. The confirmation of Ormsby's rule is seen in the 



