16 BULLETIN OF THE NATURAL HISTORY SOCIETY. 



proportion to weight. This principle is worked out in 

 various ways as follows : 



1. Various more or less light vegetative parts capable of 

 reproduction may be blown from a plant by strong winds : such 

 as leaves in begonias and Bryophyllum, joints of stem as in 

 some cactuses, rosette branches in some sedums, small bull)lets in 

 some lillies and ferns, even brittle twigs as in willows, though 

 here doubtless the mode of locomotion is rather incidental than 

 developed. 



2. Plants or their parts develop large surfaces and are 

 rolled by the wind along the ground. Entire plants curling 

 their branches upward assume a ball shape, loosen their hold on 

 the soil by rotting or breaking or pulling out of the roots and 

 are blown along, rolling over the ground as "tumble weeds," 

 either scattering their seeds as they roll, or more commonly 

 •coming to rest and scattering them, when rain gives proper 

 conditions for their growth. Such plants are rare or wanting in 

 wooded regions, but are abundant on open plains especially 

 those with a long dry season. The " Russian thistle," now 

 doing so much damage in the west, is a good tumble-weed, and 

 JPlantago cretica of Europe, and possibly the " Resurrection 

 Plant," of the south-west is another. The classical case of the 

 " Rose of Jericho," of Palestine, must be given up as it only 

 rarely and accidently becomes uprooted. It is frequently the 

 case that fruit clusters become tumble-weeds, as in some 

 XJmbellifera?, and in some clovers where the sterile stalks of a 

 head become feathered and this gives a surface for the wind to 

 roll along the fertile ones. Sometimes the fruit curls up to a 

 helix with very flat spirals, and is thus rolled along as in some 

 species of Jledicago, and there is perhaps an approach to this 

 structure in the pods of the honey locusts ; or the single fruit 

 may become ver}^ large and loose in texture, or the pod very flat 

 and thin, and be carried far from the plant by the wind and then 

 rolled along. It is probable that some seeds, as those of the 

 birches, are adapted to being blown along the surface of the 

 .snow. 



