1362 A TEXTBOOK OF THEORETICAL BOTANY 



cell", as it is sometimes erroneously called. There is a thin, tri-radiate 

 lamella between the special walls, separating the young pollen grains, and 

 this lamella may survive the breakdown of the outer walls, both general 

 and special, which finally liberates the grains at maturity. 



Although the division of the mother cell normally produces a tetrad of 

 microspores, some twenty-five to thirty species have been recorded in which 

 supernumerary microspores occur. In Fuchsia, for example, Beer has 

 observed numbers varying from five to fourteen in one mother cell. He 

 shows that they arise from irregularities at the anaphase of the first meiotic 

 division, some chromosomes lagging on the spindle and becoming separated 

 from the main groups. These separate chromosomes usually form distinct 

 nuclei and although they may contain as few as two or even a single chromo- 

 some, they undergo a second division along with the main groups. An 

 interesting point is that these imperfect nuclei produce microspores which, 

 except in point of size, are exactly like the normal grains both in structure 

 and staining reactions. It is not known whether they are ever capable of 

 functioning sexually. 



In some families the wall of the pollen grain is very slightly thickened 

 and remains undifferentiated; e.g., Orchidaceae and Xaiadaceae. The whole 

 thickness of the wall of these grains takes part in forming the pollen tube. 

 In the second group of cases the wall may thicken considerably and two 

 layers are differentiated, an external coloured layer and an internal colourless 

 layer, which are, however, continuous with one another and form only one 

 wall; e.g., Allium, Cohoea, Senecio. Here also the pollen tube is a direct 

 prolongation of the whole wall thickness. 



The great majority of pollen grains, however, have two distinct walls, 

 the extine (or exine) and the intine, but the latter alone takes part in 

 forming the tube. 



The extine appears at first as a delicate new membrane, in contact with 

 the special wall. Later, it thickens considerably and becomes differentiated 

 into two layers, the exo-extine (Sexine of Erdtman or Ferine of Kerner) 

 and the endo-extine (Nexine of Erdtman). The exo-extine is thin, has a 

 high refractive index and is difficult to see. Its outer surface is at first quite 

 smooth and the growth of spines or other appendages is a later superficial 

 development, only taking place after the endo-extine has developed, cutting 

 off the exo-extine from contact with the internal protoplasm (Fig. 1267). 

 The exo-extine envelops the whole grain, though in some cases it takes the 

 form of a delicate reticulum rather than a continuous membrane. Over 

 the pores or grooves of the grain, at which the pollen tube may appear, the 

 exo-extine is cutinized. 



The endo-extine is thick and uniform. It is cutinized throughout and is 

 continuous, except below the pores, where it is perforated by pits or canals 

 leading to the pores. 



The extine of pollen grains and the walls of fungal spores contain a 

 peculiar, highly polymerized, cyclic alcohol, called sporopollenin by Frey 

 Wyssling. It is related to suberin and cutin but is more resistant than either. 



