MORPHOLOGY OF REPRODUCTIYE ORGANS 183 



are polystemonous, the stamens nearest the centre may be 

 longer than those at the circumference, as in Luhea pcmiculata 

 (fig. 381) ; or the reverse may be the case, as in many of the 

 Rosacese. In the case of diplostemonous flowers, such as the 

 Willow Herb (Epilobium), the stamens alternating with the 

 petals are almost always longer than those opposite to them. 

 When the stamens are of two definite lengths in different fiowers 

 of the same species, as in the Primrose, tlae latter are said to be 

 dimorphic. This condition will be afterwards alluded to in 

 speaking of fertilisation. 



When there is a definite relation existing between the long 

 and short stamens with respect to number, certain names are 



Fig. 381. 



Fig. 382. 



Fig. 383. 



Fuj. 381. One of the bundles of stamens of Luhca panictdata ; the inner 

 stamens on the right are longer than the others, and are provided with 



anthers ; the shorter stamens are generally sterile. Fig. 382. Tetra- 



dynamous stamens of the Wallflower ( OheirantJms C/ieiri). Fig. 383. 



Didynamous stamens of the Foxglove i^Digitalis purpurea). 



applied to indicate such forms of regularity. Thus in the 

 Wallflower (fig. 382), and Cruciferous plants generally, there 

 are six stamens to the flower, of which four are long and 

 arranged in pairs opposite to each other ; alternating with these 

 are two solitary shorter ones ; to such an arrangement we apply 

 the term tetradynamous. When there are but four stamens, 

 of which two are long and two short, as in Labiate plants 

 generally (fiffs. 311 and '313), and in the Foxglove (fig. 383), 

 and most other Scrophulariaceous plants, they are said to be 

 didynamous. 



3. The Pollen. — The pollen consists of microscopic cells, 

 which correspond to the microspores of the higher Cryptogams. 

 They possess two coats, known as the extine and the inline. In 



