396 



THE FORMS OF TISSUES 



[CH. 



of a boundary-surface, which, whether it become at once a solid 

 or semi-soHd partition or whether it remain semi-fluid, exercises 

 in all cases an efiect on the position and the form of the boundary 

 which comes into being with the next act of division. In contrast 

 to this general process stands the phenomenon known as "free 

 cell-formation," in which, out of a common mass of protoplasm, 

 a number of separate cells are simultaneously, or all but simul- 

 taneously, differentiated. In a number of cases it happens that, 

 to begin with, a number of "mother-cells" are formed simul- 

 taneously, and each of these divides, by two successive divisions. 



Fig. 180. Various pollen-grains and spores (after Berthold, Campbell, Goebel 

 and others). (1) Epilobium; (2) Passiflora; (3) Neottia; (4) Periploca 

 graeca; (5) Apocynum; (6) Erica; (7) Spore of Osmunda; (8) Tetraspore of 

 Callithamnion. 



into four "daughter-cells." These daughter-cells will tend to group 

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

 four cells corresponding to the angles of a regular tetrahedron. 

 For the system of four bodies is evidently here in perfect symmetry ; 

 the partition-walls and their respective edges meet at equal 

 angles : three walls everywhere meeting in an edge, and the four 

 edges converging to a point in the geometrical centre of the 

 system. This is the typical mode of development of pollen- 

 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 



