The President's Address. By Lord Avebury. 283 



a long central receptacle, and curl upwards, with a long elastic awn, 

 which at maturity detaches itself from the beak elastically and 

 throws the seed (sometimes with, sometimes without the carpel) to 

 a distance of several feet. 



Even in Geranium itself the differences are considerable. 

 After the flower has faded the central axis gradually elongates. 

 The seeds, five in number, are situated at the base of the column, 

 each being inclosed in a capsule, which terminates upwards in a 

 rod-like portion, which at first forms part of the central axis, but 

 gradually detaches itself. When the seeds are ripe the ovary 

 raises itself into an upright position ; the outer layers of the rod- 

 like termination of the seed-capsule come to be in a state of great 

 tension, and eventually detach the rod with a jerk, and thus throw 

 the seed some little distance. 



In some species (G. Robertianum, G. lucidum, G. molle, 

 G. pusillum, G. 2?yrenaicum) the carpels detach themselves and 

 are thrown with the seeds. In others {G. sanguineum, G.pratense, 

 G. sylvaticum, G. columbinum, G. dissectum) the capsules remain 

 attached to the awn. The seeds are retained temporarily in place 

 by a tuft of hair. 



In this genus we get a clue to the meaning of the difference of 

 the texture of the surface of seeds. In the first group, where the 

 valves are thrown with the seeds, the surface of the seeds is smooth. 

 In the second they are more or less reticulated, which would make 

 them lighter and more easily carried by wind. It might also serve 

 to hold the seeds to the ground, and thus facilitate the exit of the 

 cotyledons. 



In Erodium the structure is somewhat similar, but the modus 

 operandi is very different. The capsules remain attached to the 

 awns, and closely envelop the seeds. The awns are twisted, and 

 more or less hygroscopic. Consequently, like those of some 

 grasses — the so-called " live oats " for instance, they elongate and 

 contract with differences in humidity. This tends to press them 

 into loose sand or earth, and as the seeds are more or less covered 

 with backward-pointing hairs, they can practically only move in 

 one direction, so that they are forced more and more deeply into 

 the ground. 



The seeds remain in the carpel, and, as in the Geranium, where 

 this is the case, they are smooth. 



In Oxalis also the seeds are thrown, but the mechanism is quite 

 different. The force resides in the seed itself. The capsule, as in 

 the preceding genera, is 5-chambered, but the walls are fleshy, 

 except opposite the middle of each chamber, where they are com- 

 paratively thin. The outer coat of each seed is a transparent 

 covering, within which is a smooth, hard black testa. The outer 

 coat contains four to five layers of parenchymatous cells. The cells 

 of the inner layer are smaller than those of the outer, closely com- 



