THE MALE GAMETOPHYTE 131 



After the two divisions, each of the four young microspores 

 becomes invested by a delicate wall which is independent of the 

 common wall of the mother-cell. This wall soon becomes differ- 

 entiated into two layers, the inner one (intine) consisting of 

 pure cellulose and later developing the pollen-tube. 



The outer layer (exine) is cutinized, and especially among 

 Dicotyledons becomes variously sculptured, often being covered 

 with ridges, w r arts, spines, etc., as fully described by Schacht 2 

 and Luerssen. 5 For the most part, there are thin spots in the 

 exine for the exit of pollen-tubes. It is interesting to note that 

 only a single point of exit occurs in the microspores of most 

 Monocotyledons and of a few Dicotyledons ; while in most Di- 

 cotyledons there are from two to many such points of exit. 

 Goebel 16 (p. 367) has given the following 

 illustrations from Schacht 2 : two points of 

 exit in Ficus, Justicia, etc. ; three in Cupu- 

 liferae, Proteaceae, Geraniaceae, Onagra- 

 ceae, Boraginaceae, and Compositae; four 

 to six in Alnus, Carpinus, Astrapaea, and 

 Impatiens; many in Alsineae, Malvaceae, 

 Convolvulaceae, etc. Barnes 13 records 



, , , . , . j. ..- 



three to twelve thin spots in the exine of dum , Muitipoiarspin- 

 Campanula, and Coulter 37 finds fifteen to die at first division 

 thirty such areas in that of Ranunculus. of pollen mother-cell ; 



. , x 400. After BELA- 



ln certain cases a much more specialized JE FF." 

 method for the exit of the pollen-tube is 

 provided, as among the Cucurbitaceae and in Passiflora, in 

 which roundish, lid-like, and often embossed pieces of the exine 

 become detached; and in Thunbergia, in which the layer of 

 exine splits into exfoliating spiral bands. Among those aquatics 

 that pollinate under water, as well as in the pollinia-bearing 

 forms, the .exine is said to be lacking. The origin and devel- 

 opment of the walls of spores is a problem that needs further 

 investigation. 



For the most part, the microspores become entirely free 

 from one another at maturity, forming a pulverulent mass, but 

 there are instances of microspores failing to become dissociated, 

 giving rise to the so-called " compound grains " (Figs. 13, 57). 

 In the simplest cases the four spores of a tetrad cling together, 

 as in Typha, certain orchids (as Neottia), Anona, Fourcroya, 



