VIII] THE SHAPES OF POLLEN-GRAINS 397 



they lie, the pollen-grains afterwards fall apart, and their in- 

 dividual form will depend upon whether or no their walls have 

 solidified before this hberation takes place. 

 For if not, then the separate grains will be 

 free to assume a spherical form as a con- 

 sequence of their own individual and un- 

 restricted growth ; but if they become solid 

 or rigid prior to the separation of the 

 tetrad, then they will conserve more or less 

 completely the plane interfaces and sharp Fig. 18 1. Dividing spore 

 angles of the elements of the tetrahedron. gi^pbdlT'' ^^^*''' 

 The latter is the case, for instance, in 

 the pollen-grains of Epilobium (Fig. 180, 1) and in many 

 others. In the Passion-flower (2) we have an intermediate 

 condition: where w^e can still see an indication of the facets 

 where the grains abutted on one another in the tetrad, but 

 the plane faces have been swollen by growth into spheroidal or 

 spherical surfaces. It is obvious that there may easily be cases 

 where the tetrads of daughter-cells are prevented from assuming 

 the tetrahedral form : cases, that is to say, where the four cells 

 are forced and crushed into one plane. The figures given by 

 Goebel of the development of the pollen of Neottia (3, a-e : all 

 the figures referring to grains taken from a single anther), illustrate 

 this to perfection; and it will be seen that, when the four cells 

 lie in a plane, they conform exactly to our typical diagram of the 

 first four cells in a segmenting ovum. Occasionally, though the 

 four cells lie in a plane, the diagram seems to fail us, for the cells 

 appear to meet in a simple cross (as in 5) ; but here we soon 

 perceive that the cells are not in complete interfacial contact, 

 but are kept apart by a httle intervening drop of fluid or bubble 

 of air. The spores of ferns (7) develop in very much the same 

 way as pollen-grains ; and they also very often retain traces of 

 the shape which they assumed as members of a tetrahedral figure. 

 Among the " tetraspores " (8) of the Florideae, or Red Seaweeds, 

 we have a phenomenon which is in every respect analogous. 



Here again it is obvious that, apart from differences in actual 

 magnitude, and apart from superficial or "accidental" differences 

 (referable to other physical phenomena) in the way of colour. 



