630 THE FORMS OF TISSUES [ch. 



when the daughter-cells remain, for a time at least, within the 

 envelope of the mother-cell. It sometimes happens, to begin with, 

 that a number of mother-cells are formed simultaneously, and 

 that the content of each divides, by successive divisions, into four 

 "daughter-cells." These daughter-cells tend to groi^p themselves, 

 just as would four soap-bubbles, into a "tetrad," the four cells 

 forming a spherical tetrahedron. For the system of four bodies 

 is in perfect symmetry. The four" cells are closely packed within 

 the cell-wall of the mother-cell ; their outer walls divide the 

 sphere into four equiangular triangles; their inner walls meet 

 three-by-three in an edge, and the four edges converge in the 

 geometrical centre of the system; and these partition walls and 

 their respective edges meet one another everywhere at co-equal 

 angles. This is the typical mode of development of pbllen-grains, 

 common among monocotyledons and all but universal among 

 dicotyledonous plants. By a loosening of the surrounding tissue 

 and an expansion of the cavity, or anther-cell, in which they He, 

 the. pollen-grains afterwards fall apart, and their individual form 

 will depend upon whether or no their walls have soKdified before 

 this liberation takes place. For if not, then the separate grains will 

 be free to assume a spherical form as a consequence of their own 

 individual and unrestricted growth ; but if they become set or rigid 

 prior to the separation of the tetrad, then they will conserve more or 

 less completely the plane interfaces and sharp angles of the elements 

 of the tetrahedron. The latter is apparently the case in the pollen- 

 grains of Epilobium (Fig. 278, 1 ) and in many others. In the passion- 

 flower (2) we have an intermediate condition : in which we 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. In heaths and in azaleas the four 

 cells of the tetrad remain attached together, and form a compound 

 tetrahedral pollen-grain. Six furrows correspond to the six edges 

 of the tetrahedron, and each is continued across a pair of cells; they 

 are formed (I take it) along lines of weakness at the edges of the 

 tetrahedron, and they make three furrows upon each one of the 

 four coherent grains, just as we see them on a large number of 

 ordinary separate and non-coherent pollen-grains. On the other 

 hand, there may easily be cases where the tetrads of daughter-cells 



