THE ANGIOSPERMAE 



1367 



Protein deposits in crystalline form are also sometimes present, as for 

 example conspicuously in the pollen grains and pollen tubes of Asclepia- 

 daceae. Anthocyan colouring substances are not uncommon and many types 

 of pollen are red or purple in consequence. The green pollen of Lythrum 

 (see p. 1278) is not due to chlorophyll, which has not been recorded in pollen, 

 although plastids are present, but to a mixture of a blue anthocyan with a 

 yellow pigment. The former pigment turns red in contact with the acid 

 secretion of the stigma. Some pollens contain aromatic substances which 

 may affect the choice of pollen by collecting bees. The aromatic pollen of 

 Acorus is sold in Iraq as a flavouring substance for cakes. 



The external surfaces of pollen grains bear a great variety of markings, 

 though a few are quite smooth, e.g., some Gramineae. These are described 

 as psilate, but they are rare except among Gymnosperms. The exo-extine 

 or sexine is usually formed of pila, little processes like drumsticks, with a 

 stalk, the bacuhim and a head, the caput. The capita are often confluent, form- 

 ing a roof or tegillum, which may also be formed by a membrane covering the 

 capita. The tegillum is often punctate with small holes. Spines, granules, 

 warts and other forms of ornamentation may be produced superficially on 

 the tegillum. Bladder-like expansions involve the separation of the pila 

 from the nexine. Reticulations, striations and other patterns on the surface 

 of the grain are due to special arrangements of the pila (Fig. 1270). These 

 are all protrusions from the extine surface, but there may also be pits or a 

 negative reticulation of grooves sunk in the wall. 



Wodehouse has suggested a physical theory to account for at least some 



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W&i 



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E F G H 



Fig. 1270. — Pollen sculpturing. A, Pilate. B, Reticulate. C, Striate. D, Ornate. E, Crassi- 

 sexinous. F, Tenuisexinous. G, Tegillate. H (on left), Subsaccate ; (on right), Tegil- 

 late, verrucous and spinose. {After Erdtman.) 



L* 



