1376 A TEXTBOOK OF THEORETICAL BOTANY 



monocolpate pollen, which is common in Gymnosperms and Monocoty 

 ledons, is confined to certain Ranales and Piperales among the Dicotyle- 

 dons. Most of the Ranalean families of tree habit, including the very 

 primitive Winteraceae, have monocolpate pollen. The Ranunculaceae (with 

 few exceptions), the Berberidaceae, Menispermaceae, and the homoxylous 

 genera Trochodendron and Tetracentron are all tricolpate. The Nym- 

 phaeaceae, which according to one view, stand close to the Monocotyledons, 

 are heterogeneous, Nymphaeoideae and Cabomboideae being monocolpate 

 and Nelumbonoideae tricolpate. 



At the time when the pollen grains separate from the tetrads in which they 

 were formed, they are, for the most part, still uninucleate, but nuclear divi- 

 sion soon follows. In the Ericaceae, of course, as the tetrads are permanent, 

 mitosis takes place in that condition. The same is naturally true of composite 

 pollens in Mimosaceae, Orchidaceae, etc. Some types in which the pollen 

 eventually separates into single grains, however, regularly undergo the 

 first mitosis before separation. Such are Elodea, Salpiglossis simiata and 

 members of the Juncaceae. 



Fig. 1279. — Cuscutci epithymum. Left: pollen grain with ge;ierative nucleus in 

 mitosis. Right: ripe three-nucleate grain with small prothallial (?) cell. 

 {After Fedortschiik.) 



A few isolated observations have been made of a preliminary division 

 leading to the formation of a small cell at one side of the pollen grain, before 

 the mitosis which gives rise to the generative nucleus (Fig. 1279). These 

 cases, in Cusaita, Liliiim, Eichhornia, Atriplex and Sparganhim, are too rare 

 to be regarded as normal occurrences, but, if the facts are correct, the 

 suggestion that this is a true prothallial cell, comparable with those normally 

 formed in the pollen of Gymnosperms, is certainly very interesting. 



There is not usually any long interval between the separation of the free 

 microspores and the onset of mitosis, but in some spring-flowering species, 

 where the floral organs are preformed in the late autumn, the grains may 

 pass the winter in the uninucleate condition. Tropical plants have no 

 resting period, but it tends to lengthen with increasing distance from the 



