SPORE LIBERATION 



(Rhinanthaceae), and the heaths Calluna and Erica — but not Rhododendron^ 

 which has very sticky pollen. 



(iv) Tropical and sub-tropical trees include few anemophilous species, 

 but Casuarina and Myrothamnus are wind-pollinated, and some of the 

 palms, although entomophilous, shed a good deal of pollen, which may be 

 carried by the wind. 



(v) Nettles and their allies form an anemophilous group which do not 

 store pollen after dehiscence of the anthers. The anthers dry as they 

 mature, tissue tensions are set up, and suddenly, as the pollen sacs burst, 

 the filaments uncoil, throwing pollen into the air. The process can be 

 watched on a still, dry day when small puffs of pollen appear as the nettle 

 flowers explode, but in damp air dehiscence of the anthers is inhibited. 

 The mechanism occurs in Urtica, Parietaria, Moms, and Broussonetia. 

 A sifter mechanism similar to that of the grasses occurs in Cannabis and 

 Humiilus. 



(vi) Herbs with inflorescences elevated above the general level of the 

 foliage include a number of anemophilous types such as Mercurialis. 

 In Plantago and Globularia the anthers, w^hich are exposed in cups, close 

 their slits in moist weather but shed their pollen in dry air. Upward- 

 facing cups occur also in Poterium and Sanguisorba. Sifter mechanisms 

 occur in some species oi Rimiex and Thalictrum. Other conspicuous pollen 

 shedders occur in the Chenopodiaceae {Beta, Salsola, Chenopodium) and 

 in the Amaranthaceae, and also in some groups within the Compositae — 

 especially Ambrosia and Artemisia. 



(vii) Deciduous trees of temperate regions form a biological group. 

 Typically the male flowers are aggregated into pendulous catkins, usually 

 appearing shortly before the leaves expand. In Alnus, Betula, Castanea, 

 Corylus, Fagus,Juglans, Populus {Salix, like Tilia, is both insect-pollinated 

 and a wind shedder), and Qtiercus, pollen is protected from rain after 

 shedding while temporarily stored on the upper scales of the flower 

 standing underneath — until it is blown away by w ind in a manner reminis- 

 cent of Pi?ius. Platanus closes its catkins by a hygroscopic mechanism, 

 so that pollen is not merely protected from rain but can be blown away 

 only in dry weather. Hippophae pollen is shed into the base of the flower 

 while this is still in bud. At maturity the perianth lobes remain united at 

 the top but separate at the base, leaving slits through which pollen can 

 be removed by the wind. In another group, including Fraxinus, Buxus, 

 Phillyrea and Ulmus, the anthers project as upward-facing cups from which 

 pollen is removed by wind. 



The take-off mechanisms briefly sketched in this Chapter, with others 

 (doubtless including some still undiscovered), are not mere curiosities of 

 natural history. On the contrary, they are highly efficient processes that 

 restrict spore liberation to limited meteorological conditions. Pollen and 

 spores of mosses and ferns tend to be shed into dry winds. Ascomycetes 



43 



