X] POLLEN-GRAINS 87 



other parts of the flower: in this case they are said to 

 be ob-diplostemonous. 



Stamens grouped in bundles, whether by branching 

 or by coalescence of the filaments, are monadelphous, 

 diadelphous, to polyadelphous, according to the number 

 of bundles. 



In cases where the anthers are coalescent they are 

 termed syngenesious, e.g. Composite, Cucurbitacea3, Lobelia. 



In some cases the fusion is still more complete, com- 

 prising not only cohesion of stamen and stamen, but also 

 adhesion with the gynoecium. The andrcecium is then 

 termed gynandrous; e.g. Orchids, Asclepiads, Aristolo- 

 chiacese, &c. 



Where the fusion occurs with the calyx-tube, the 

 stamens are, as we have seen, perigynous or epigynous\ 

 and where with the petals or corolla, epipetalous. The 

 stamen opposite a petal, but not necessarily joined to it, 

 is often termed antipetalous, and so on. 



Pollen is not always, as the name would imply if we 

 observed its etymology, a powdery dust, though it is so in 

 the majority of cases, and each species of pollen-grain has 

 its definite size, shape, colour, and ornamentation, the 

 diameter varying from about 25 //, to 250 //,, and being more 

 or less spheroidal or ellipsoidal, yellow, and covered with 

 spicate or knob-like or other projections, to facilitate the 

 attachment of the grain to insects, the stigma, &c. There 

 is often no relation discoverable between the external 

 peculiarities of the pollen-grain and the systematic posi- 

 tion of the plant ; but there is between the mode of 

 pollination and these peculiarities, and it is only when 

 all or most of the members of a group are pollinated in 

 the same way that the former relationship exists. 



As examples of pollen-grains the student may compare 

 and contrast the following. The large spherical grains of 



