levine: somatic and reduction divisions in drosera 139 



fully developed pollen tetrads. My studies were made on paraffin 

 sections and on tetrads mounted in toto in glycerine to which a 

 few drops of glacial acetic acid were added. The pollen grains in 

 all the species I studied are arranged most commonly in tetra- 

 hedral form as were the nuclei in each of the pollen mother-cells 

 just before cell division. Each tetrahedron of course shows three 

 of the grains in one plane, while the fourth is hidden by them. 

 All the contact walls of course intersect at an angle of 120°. Be- 

 tween each two adjacent pollen grains a double row of projecting 

 tubules may be seen, one row coming from each grain. These 

 have been described by Drude as young pollen tubes. In my 

 opinion they are to be regarded as highly specialized germ pores 

 as will be further discussed below. In another less common type 

 found in the Droseras the pollen grains are arranged in pairs; a 

 line through the center of one pair lying at right angles to a similar 

 line through the other pair, and in a different plane. This arrange- 

 ment of the grains is shown in Fig. 42. A similar arrangement 

 of the grains has been figured by Andrews ('05) for Epigaea repens 

 L. The rows of tubules are also present with the grains in this 

 arrangement. So far as I can find, this particular type of germ 

 pore is peculiar to the Droseras and it is certainly worthy of 

 more careful study than has yet been given to it. 



Each surface of the four grains in a tetrahedron is triangular 

 with one side more or less convex, which forms the base of the 

 grain. The other three lateral faces are contact surfaces by which 

 the pollen grain is joined to its neighbor in the tetrad. The exine 

 of the external convex surface of each grain is thickly covered with 

 short, almost tubercular spines as shown by Rosenberg ('09). 

 The tips of the spines are occasionally ellipsoidal or lance-shaped. 

 The exine of the contact walls of the grains is somewhat thinner 

 than that of the external surface. On the inner surfaces of the 

 contact walls are found the so-called "tubules" which constitute 

 as already noted the most peculiar characteristics of the pollen 

 grain of the Droseraceae. They arise at the time when the thicken- 

 ing of the wall is going on and constitute in effect a diverging series 

 of channels or tubules lying on the inner surface of each of the 

 three contact walls of a grain. As shown in text-figure i, 

 they take their origin near the inner angle of each contact wall, 

 that is, near the apex of the tetrahedral pollen grain, considering 



