4 6o SCIENCE PROGRESS 



are afforded by many fruits, as for example the gorse, in which 

 the valves separate suddenly from each other and curl up, thus 

 causing the seeds to be flicked out to a considerable distance. 

 The violet again exhibits a similar device, though the details 

 differ. The opening of the three little windows in a campanula 

 fruit is effected by small strips of tissue situated about equi- 

 distantly around the periphery of the capsule wall. The strips 

 can be identified even in the unripe fruit as occupying a position 

 at the bottom of three corresponding depressions in the wall. 

 When the fruit ripens, they become torn away from the rest of 

 the wall and roll backwards into the fruit cavity, thus leaving 

 the openings and windows through which the seeds escape. 

 When damp conditions supervene, each flap uncurls and closes 

 up its corresponding aperture. This movement of opening and 

 closing can be repeated many times with the same fruit and is due 

 to the circumstance that the cell walls near the inner surface 

 absorb (and give off) more water than those of the outer surface. 

 Another interesting example is afforded by the curious cruci- 

 ferous plant known as the Rose of Jericho (Anastatica). When 

 its seed pods are ripe, the plant dries up and its branches curl 

 hygroscopically inwards ; but when wetted, they again unfold 

 and the fruits open and scatter the seeds ; the plant remains 

 rooted in the ground all the time and is not blown over desert 

 sands, as has often been fancifully stated. Investigation of the 

 structure of the stem shows that, though the wet and dry 

 positions respectively are the reverse of those often assumed 

 in the case of other plants, e.g. the thistle bracts mentioned 

 above, nevertheless the actual mechanism is identical in principle 

 in each case. A transverse section of the stem shows that the 

 wood on the upper side of a branch of the Rose of Jericho 

 differs from that on the lower side in containing far fewer 

 vessels and considerably more fibrous tissue. Further examina- 

 tion of the fibres themselves on the upper and lower side of the 

 stem respectively reveals other and significant differences of 

 structure. The walls of the fibres are very thick and they show 

 pitted markings on their surface ; but whilst the general 

 direction of the slit-like pits of the upper fibres is transverse, 

 that of the pits on the lower fibres is mainly longitudinal or 

 oblique. Now the arrangement of the pits affords a clue to the 

 direction of greatest swelling. The wall behaves as if it were 

 built up of brick-like particles (micellae) between which water 



