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A TEXTBOOK OF THEORETICAL BOTANY 



important method of distribution in our native species. It should be noted 

 that the very similar parachute cup in Armeria is formed by the sepals. 



Many of the Gramineae drop their caryopses enveloped in the glumes 

 or they may abstrict whole spikelets as units of dispersal. Definite layers 

 of abstriction tissue are formed, the cells of which either break or separate 

 easily from one another, causing abstriction. This may occur above the 

 empty glume (Agrosteae), the fruit remaining encased by the flowering 

 glume and the palea; or, the spikelet may fall as a whole (Paniceae) with 

 both glumes; or, if the spikelets form a spike (Hordeae), the rachis of the 

 latter may disarticulate, separating into joints each with one spikelet. 

 Whichever method is used, the attachment of the glumes increases the 

 chances of wind carriage. The frequent occurrence of grass plants on 

 walls at some height above ground shows that the method is successful. 



A curious case of wing formation is shown by Paliurus (Fig. 141 5), in 

 which it is the enlarged disc of the flower which forms a parachute for 

 wind dispersal. 



Fig. 1416. — Clematis vit alba. Fruiting shoot showing 

 the long, persistent styles which are becoming 

 plumose. 



