62 VEGETABLE ORGANOGRAPHY. 



in compact masses, as in the Orchideas and Asclepi- 

 adeas. 



The surface of the grains presents an important 

 difference : sometimes it is perfectly smooth and not 

 viscid, as in Justicia quadrifida and a multitude of other 

 plants : sometimes it is invested with a viscid layer 

 differently coloured, and which appears to be produced 

 by true secreting organs ; we see this in most of the 

 Cinarocephalae, Heliantheae, &c. : the surface of some 

 bears mamilliform eminences, as in Cobeea : in the 

 Cucurbitaceas it is furnished with pointed projections, 

 which Amici considered as kinds of lids terminating in 

 a point. 



The general form of the grains of pollen is most 

 frequently globular, oval, or elliptical in Dicotyledons, 

 and lengthened elliptical in Monocotyledons. It is 

 frequently remarked, that the oval or elliptical ones 

 are marked on one side with a longitudinal furrow, 

 resembling that in a grain of wheat. They are fre- 

 quently found polyhedral, as in the Chicoraceas ; some 

 are nearly triangular, as those of the Proteaceaa and 

 Onagrariae ; those of Colutea are oval and truncated at 

 the extremities. Among elliptical or elongated pollens 

 there are sometimes found curved ones : Grew has even 

 mentioned ramified ones, but this observation does not 

 appear to have been verified. In general, plants of the 

 same family have their pollen nearly alike ; but analogous 

 forms are found in very different families. 



Each grain of pollen contains a fluid which appears 

 to be slightly -viscid, and has been named the Fovilla ; 

 it is the true fecundating liquor of plants, and is full of 

 little grains, endowed with a peculiar motion, and which 

 are called Granules. 



The pollen grains open at maturity either by a 

 regular dehiscence, or, most usually, by an irregular 





