632 THE FORMS OF TISSUES [ch. 



the four cells remain attached to one another, and lie upon a level 

 with a " polar furrow " well displayed. Occasionally, though the four 

 cells lie in a plane, the diagran^^seenis 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) for the most part develop in much the same way as pollen- 

 grains; they also very often retain traces of the shape which they 

 assumed as members of a tetrahedral figure, and the same is equally 

 true of liverworts. Among the " tetraspores " (8) of the Florideae, 

 or red seaweeds, we have a condition which is in every respect 

 analogous. The same thing happens in certain simple algae alHed to 

 Protococcus: where four daughter-cells, confined within a mother- 

 cell, form a spherical tetrahedron, much hke a spore of Osmunda on 

 a smaller scale*. 



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, 

 texture and minute sculpture or pattern, a very small number of 

 diagrammatic figures will sufficiently represent the outward forms 

 of all the tetraspores, four- celled pollen-grains, and other four- 

 celled aggregates which are known or are even capable of existence. 

 And it is equally obvious that the resemblance of these things, to 

 this extent, is a matter of physical and mathematical symmetry, and 

 carries no proof of near relationship or common ancestry. 



We have been dealing hitherto (save for some slight exceptions) 

 with the partitioning of cells on the assumption that the system 

 either remains unaltered in size or else that growth has proceeded 

 uniformly in all directions. But we extend the scope of our enquiry 

 greatly when we begin to deal with unequal growth, with cells so 

 growing and dividing as to produce a greater extension along 

 some one axis than another. And here we come close in touch 

 with that great and still (as I think) insufficiently appreciated 

 generahsation of Sachs, that the manner in which the cells divide 

 is the result, and not the cause, of the form of the dividing structure : 

 that the form of the mass is caused by its growth as a whole, and 



* Cf. A. Pascher, Arch. f. Protozoenk. lxxvi, p. 409; lxxvii, p. 195, 1932. 



