i5i8 A TEXTBOOK OF THEORETICAL BOTANY 



woolly seeds. These are Bombax, Ochroma, Eriodendron and Gossypium. 

 The latter is the Cotton plant and the seed-hairs need no description, 

 while Eriodendron anfractuosum produces hairs which form the Kapok of 

 commerce. Ochroma lagopus is the Balsa tree. 



There are two other aspects of the wind dispersal of seeds which do 

 not depend on any special modification of the seed itself. One is the 

 shaking of small seeds out of open capsules by the force of the wind swaying 

 the plant. This has sometimes been called the "censer mechanism". 

 It depends for its effectiveness on the dehiscence of the capsules at or near 

 the top, so that the seeds do not naturally fall out, but require a sharp 

 jerk to dislodge them. The classic example is the capsule of Papaver on 

 its long, springy pedicel, but Digitalis, Campanula, Verbascum and Oeno- 

 thera are equally good instances and many more could be cited, for it is a 

 common method among tall herbs. A curious feature related to this 

 method is seen in Campanula. The capsules of C. rapunculus are held 

 upright and the small apertures of dehiscence are at the top, as also in 

 Antirrhinum, but in C. rapunculoides the capsules are pendent and the 

 openings in this case are at the morphological base of the capsule, which 

 is here the upper end. Most seeds dispersed in this way are small and as 

 they are only discharged in high winds they can also be carried several 

 yards by the wind itself. 



The second case is that of "tumble weeds". The whole plant dies 

 when the seeds are ripe and is readily blown off the ground and rolled 

 along, dropping the seeds as it goes. This is a peculiarity of plants growing 

 on open plains or prairies, where it is most effective. The tumble weeds of 

 the North American prairies. Sisymbrium altissimum (Cruciferae) and 

 Salsola kali (Chenopodiaceae), the "Russian Thistle", although both 

 introduced plants, have become widespread and noxious weeds through 

 this habit. In some species it is not the whole plant but only the infruc- 

 tescence which is detached and blown away. Several of the Clovers 

 {Trifolium globosum and T. subterraneum) are thus distributed and the 

 method is not uncommon among grasses. A famous example in the latter 

 family is Spinifex squarrosus, together with some other species of this sea- 

 shore grass, which range all over S.E. Asia and Australasia. The female 

 spikelets form a round head, with greatly elongated bracts ending in spines 

 (Fig. 1388). When this head breaks off it rolls on the tips of the spines and 

 travels at a surprising speed, dropping seeds as it goes. It is so light that 

 it can also float and drift on water. The light panicles of several other dry- 

 land grasses are rolled about in a similar way after being detached from the 

 plant. Clumps of thistle-dow^n may also be borne or rolled by the wind 

 for great distances over both sea and land, but the fruits do not often re- 

 main for long attached to them. 



One of the so-called " Rose of Jericho " plants is the little annual 

 Crucifer, Anastatica hierochuntica, which rolls up its branches into a ball 

 when the seeds are ripe and the plant dries up. The mass of fruit-bearing 

 branches is blown about by the wind, but in this plant the branches spread 



