4] DISPERSAL AND MIGRATION 



lOI 



tion effects of high aUitiides. According to Ridley, whose monu- 

 mental work on plant dispersal is cited at the end of this chapter, 

 ' There is no part of the world where some are not present, and 

 there appears to be a constant rain of the more minute kinds falling 

 everywhere.' With little doubt this easy wind-dispersal of many 

 of the Bacteria, Fungi, and other so-called spore-plants is the 

 primary reason for their extremely widespread distribution ; a 

 secondary reason is their often wide tolerance of conditions and 

 modest requirements for life. 



(b) Dust seeds (and minute fruits). The seeds of many plants, 

 such as the members of the Orchid family (Orchidaceae), and the 

 one-seeded fruits (for example) of some of the mainly tropical 

 parasitic family Balanophoraceae, are also minute and extremely 

 light, as well as sometimes winged, and so tend to be blown away 

 and about in much the same manner as spores. 



(c) Plumed seeds. These usually bear a light tuft of silky hairs 

 at one end and are liberated from a capsular fruit which, on splitting, 

 only releases them gradually, often one by one. The plants involved 

 are usually herbs or climbers, good examples being species of 

 Willow-herb (Epilobium) and Milkweed (Asclepias), and they gener- 

 ally occur in open situations, in or from which they can travel for 

 hundreds of miles. 



{d) Plumed fruits. These include the familiar ' parachutes ' of 

 Dandelions {Taraxacum spp.), the long feathery fruits of species 

 of Avens (Geum), and the silky-haired ones of Cotton-grasses 

 {Eriophoriim spp.). Their appendages cause them to be detached by 

 the wind and floated away, often for very considerable distances. 

 The plants concerned are usually herbaceous, and include many 

 Grasses. An extreme case is that of some disseminules of Grasses 

 which have been trapped in the air several thousands of feet above 

 the ground, and in view of the highly fortuitous nature of such 

 observation it w^ould seem likely that they may reach the upper 

 air currents quite frequently. 



{e) Winged seeds. In these it is usually a thin portion of the 

 seed-coat which forms a wing that catches in the wind when they 

 are liberated, often in considerable numbers, e.g. by splitting of 

 the containing fruit-wall. They chiefly occur on trees, shrubs, 

 and lianes (woody climbers), and so are liberated some distance 

 above the ground — which is just as well, for their dispersal mechanism 

 tends to be much less eflicient than those of the categories mentioned 

 above. Good examples are aflPorded by members of the Bignonia 



