6oo PATTERNS AND PROBLEMS OF DEVELOPMENT 



directions. Here the pores through which the spore protoplasm pushes 

 outward may serve to locahze the outgrowth and so the axiate pattern. 



Spores of bryophytes and pteridophytes differ in form, those of some 

 species being spherical, others tetrahedral because of relation to sister 

 cells, still others bilateral, and in some the spore coat (exospore) may 

 rupture in a definite manner on germination; but whether a definite axiate 

 pattern is determined in the spore protoplasm seems not to be known. 

 Germination is apparently a budding, an activation, and an outgrowth of 

 a region of the spore protoplasm, determined either by the region of 

 rupture or by some local differential; or, if the spore possesses an axiate 

 pattern, that presumably determines the region of initiation of outgrowth. 

 The primary outgrowth may be a rhizoid, development of other axes oc- 

 curring later. In the true mosses the moss plant originates as a bud from 

 the filamentous protonema which results from spore germination. This 

 type of development resembles the development of the hydranth and stem 

 in many hydroids from stolonic outgrowths. Evidently the various bud- 

 dings involved in these forms of development represent new polarities 

 originating in local activations. Like buds of higher plants and of animals, 

 they are probably primarily radial gradient systems and become axiate 

 by differential growth of central and peripheral regions. 



Spores of mosses and ferns develop from certain cells of sporangia, 

 which are usually axiate. Whether spores ever possess a polarity derived 

 from that of the sporangium seems not to be known; but if they do, it is 

 probably readily obliterated and a new polarity determined by local ex- 

 ternal conditions. Certain of the pteridophytes are heterosporous, and 

 the male gametophytes developing from microspores are reduced to a 

 few cells inclosed by the spore wall but show a definite orderly pattern, 

 suggesting a polarity. If a polarity is present, it is probably determined 

 by the relation to each other and to free surfaces of the four spores of a 

 tetrad. The female gametophyte of the heterosporous forms, Selaginella 

 and Isoetes, on the other hand, shows a definite polar pattern in general 

 coincident in direction with the polarity of the sporocarp axis on which it 

 originates (e.g., F. M. Lyon, igoi). 



In certain water ferns (Hydropterineae) an axiate sporocarp, arising 

 essentially as a bud, consists of an outer covering (indusium) inclosing a 

 branching axis with megasporangium apical and one-celled microsporangia 

 at the tips of lateral branches. Development of any of the thirty-two cells 

 in the terminal megasporangium inhibits development of the microspo- 

 rangia; but if all cells of the megasporangium degenerate, the microspo- 



