ItlANSACTlONS OP SECTION K. 855 



8, The Structure of Leaves of the Bracken from different habitats. 



By L. A. Boodle. 



The external characters of the hracken {Pteris aquilina) vary with the habitat, 

 as has been pointed out by different authors. In a very exposed and sunny 

 situation the leaves are hard and short, while in a well sheltered and shaded 

 locality they are much larger and soft. Long sori and short sori are typical of 

 the tirst and second situation respectively. 



The internal structure of the pinnules varies with the habitat in a correspond- 

 ing manner ; the presence of a continuous or nearly continuous hypoderm and the 

 large amount of the palisade tissue formed distinguish the leaf of the exposed from 

 that of the sheltered plant. 



That these difi'erences are not necessarily varietal is proved by the following 

 observations. A leaf which had grown up through a fairly dense bush of haw- 

 thorn, bramble, &c., showed, in its lower pinnfe, which were immersed in the 

 bush, the external and internal characters of the shade form, while the upper 

 pinnse, which were free from protection, had the characters of the exposed form. 

 Secondly, the relation of the diiferent characters in question to external conditions 

 was also evidenced by a plant which was grown in a greenhouse (with heat) last 

 year, and was planted out again in the autumn. In the greenhouse it produced 

 onlv leaves of the shade type, of very delicate texture, and with tlie further 

 peculiarity that both the indusia were often reduced in size. This year the same 

 plant, growing in the garden, produced leaves of the type referred to above as 

 characteristic of the braclien in an exposed situation. 



Such formation of a mesophytic or a xerophytic type of leaf by the same plant 

 in accordance with its environment is an example of what has been called ' direct 

 adaptation,' and may be compared with Bonnier's experiments on transplantation 

 to Alpine habitats and with the results of several authors on * sun-leaves ' and 

 * shade-leaves.' 



MONDAY, SEPTEMBER 14. 

 The following Papers were read : — • 



1. Discussion on the Evolution of Monocotyledons, 



i. The Evolution of Monocotyledons. By Ethel Sargant. 



Monocotyledons and dicotyledons together form a group very distinct from 

 any other, and the probability is strong that the common stock from which they 

 spring was in all essential features Angiospermous. 



Was the ancestral Angiosperm more like a monocotyledon or a dicotyledon — 

 that is, which of these two types is the more primitive ? 



An answer to this question has been sought in three distinct lines of research, 

 but these have led to no positive evidence in favour of either branch. 



I. No direct historical evidence is given by the succession of fossil forms. 



II. There is no reason to suppose the absence of a normal thickening ring in 

 the stem of monocotyledons a primitive character. 



III. The development of the embryo within the embryo-sac has not proved 

 satisfactory as a guide to affinities. 



The study of pseudo-monocotyledons such as Coi-ydalis cava, 

 Ranunculus Ficaria, Carum Bulbocastanum, has sliown that the 

 embryo within the embryo-sac in such species appears monocotyloua 

 from the first. Yet the common ancestor of each genus must have 

 been dicotylous, and if the early history of the embryo-sac were any 

 guide to race-history, we should expect it to throw light on the 

 transition from a dicotylous to a monocotylous form. 



