THE ANGIOSPERMAE 



1373 



tetrahedral, psilate grains with one prominent basal pore and three sHt- 

 like lateral pores. The Ericaceae are notable because the pollen remains 

 permanently in tetrads, a peculiarity which they share only with a few other 

 families, e.g., Empetraceae. The grains are closely united in a tetrahedron, 

 which may be almost spherical in outline (Fig. 1276). The extine is smooth 

 or finely reticular and the three furrows of each grain are short, narrow and, 

 in Calhina, poorly defined. The latter genus produces enormous quantities 

 of pollen, which may be carried by the wind. 



Fig. 1276. — Pollen grains united in permanent tetrads. A, Empetrum 

 tiignim. B, Andromeda poUfoUa. C, Kahnia latifolia. (After 

 Erdtman.) 



On the other hand, some families are markedly heterogeneous in respect 

 of their pollen. The Caryophyllaceae have, for the most part, cribellate 

 grains, that is, grains with many pori, but Spergula and a few other genera 

 have tricolpate grains. Apart from all others, however, are the Acanthaceae, 

 which show an astonishing variety of pollen, almost every type being repre- 

 sented in the family, although the type in each genus is fairly constant. 

 This is a family in which the pollen morphology may well call for systematic 

 consideration. 



Very little information is available about the effect of polyploidy on 

 pollen morphology nor has the question of variation between local popula- 

 tions of the same species been much explored, though both ot these avenues 

 of research promise interesting results. (For variation in dimorphic flowers, 

 see p. 1275.) 



The permanent cohesion of grains into groups larger than tetrads is a 

 conspicuous feature of certain families. The stamens of Mimosaceae have 

 transversely septate pollen loculi in the anthers and each segment of the 

 loculus may contain only a small number of grains. These grains are per- 

 manently united, in most species, into groups or massulae containing from 

 4 to 64 grains, the larger numbers involving all the grains in a single anther 

 segment (Fig. 1277). Among the Orchidaceae there is considerable varia- 

 tion in this respect. Cypripedium has isolated grains, Neottia and Listera 

 have grains united into tetrads. In Orchis the grains are closely united into 

 massulae containing numerous grains, and in Vatida and many other 

 Orchids all the grains in a loculus are united into one pollinium (Fig. 1278). 

 The anther of Orchis has a relatively small number of primary sporogenous 

 cells in each lobe, arranged in the arc of a circle. Each of these cells divides 

 repeatedly and gives rise to a massula of grains, whose thin walls have no 

 extine, except the grains on the outside, where there is an extine which 



