1887-88.] ^Ir Fothergill on Leaves of Clinihing Plants. 309 



a few rough lists of orders where climbers occur, I became 

 convinced that there must be some causal connection 

 between the climbing habit and a large development of the 

 basal portions of the leaves. 



Accordingly, I went through the collections in the 

 Edinburgh Herbarium, making, by aid of the " Genera 

 Plantarum," lists of climbing plants in almost every order 

 where they occur. In one column I placed those whose 

 leaves were cordate, sagittate, or hastate, as I expected, 

 and in another those not marked by this peculiarity. A 

 glance down the column of exceptions in each order gave a 

 clue to a rationale of the point, the apparently unfavourable 

 cases as usual suggesting at the same time their own explan- 

 ation and the basis for a general rule. 



The salient feature of a climbing plant is, of course, the 

 weakness of its stem. Whether this is the cause or the 

 result of the climbing habit is, of course, beyond the present 

 question. 



External observation of the most typical plants in 

 question showed that the stronger and stitfer the stem, the 

 less marked was the basal development of the leaves. A 

 series of sections confirmed the generalisation that basal 

 development varies inversely with the amount of strengthening 

 tissue in the stem. In some different specimens of ivy, for 

 example, those with no large basal leaf-lobes had much more 

 woody stems than those with markedly hastate leaves. 



A large number of cases, however, contradicted the rule 

 even as thus modified, and it was only on noticing that in all 

 the cases where the stem was soft, and yet the leaves were 

 not basally developed, they were sessile or on very short 

 stalks. The length of the petiole then appeared as a second 

 factor which is seen to vary directly ivith the amount of basal 

 development ; the longer the stalk, the more cordate or sagittate 

 the leaf. 



We can express these two modifications of the rule that 

 climbing plants have basaUy developed leaves as follows : — 

 Let b be the amount of basal development, s the amount of 

 strengthening tissue in the supporting system, and j9 the 



length of the petiole, then & co - . 



s ' 



I must note here that, where leaves- are compound or 



TRANS. BOT. SOC. VOL. XVII. X 



