478 Transactions of the Society. 



combining these two halves, brings them up to the full number again. 

 Thus the qualities of the two parents are combined in the young • 

 animal or plant, and the reason for there being as a rule an even 

 number becomes evident. 



There are, however, interesting and remarkable exceptions. As 

 already mentioned, in certain insects, for instance in Anasa, one 

 of the Heteroptera, half the spermatozoa are said to contain one 

 more chromosome than the others. 



The Form and Surface of Pollen, 



From one point of view pollen, may be divided into three main 

 classes : — 



1. Aerial pollen carried by wind. (This is probably the 



original arrangement.) 



2. Aerial pollen carried by insects. 



3. Subaqueous pollen. This is a comparatively small group. 

 Again, pollen may be divided according to form and structure 



into : — 



1. Elliptical pollen, generally smooth. 



2. Spherical pollen or hemispherical pollen, either — 



{a) Spiny, and generally carried by insects. 

 (?)) Smooth, and generally carried by wind. 



3. Elongated filiform pollen. 



There are, however, many intermediate forms — barrel-shaped, 

 square, brick-shaped {Viola hUca, figs. 13 and 14), triangular, 

 faceted, etc., and there are various abnormal and peculiar pollens. 



In some orders the pollen is fairly uniform in character, in 

 others there are great diiferences. To the former category belong, 

 among others, the Cruciferse, in which the pollen is ellipsoid, with 

 three furrows. This form prevails also among the Eesedaceae, 

 Oleacew, and many other orders. 



All the Malvaceae I know have spherical and spiny pollen. 



Other families present great differences. Among the Geraniaceaj, 

 for instance. Geranium and Erodium have spherical pollen, pass- 

 ing, however, sometimes into oblong. That of Oxalis is almost 

 barrel-shaped, some of the Inipatiens are brick-shaped, some square, 

 some dumbbell-shaped, some triangular. That of Limncmthis is 

 very peculiar, being elongated and bent into the form of a letter 

 U or V. 



Some Cucurbitacese {Brijonia) have elliptic pollen ; in others 

 (Cucurbita and Momordica) it is spherical and spiny ; while that 

 of Cueumis is triangular and finely netted. 



The Campanulacese present at least three types : — 



1. Spherical and echinulate (C'ampaw^/ff, Plri/teuma). 



2. Ellipsoid, 3-furro\ved, and smooth (Lohelia). 



3. Tumid, shortly oblong and echinulate {Platycodon). 



