CHAPTER XV 



QUESTIONS OF ORIGIN OF CERTAIN AGAMIC 



PATTERNS UNDER NATURAL 



CONDITIONS 



CONCERNING various developmental patterns we know little or 

 nothing beyond the fact of their appearance under natural con- 

 ditions. They have been described, but experimental analysis is 

 lacking. Some of them are practically inaccessible to present experimen- 

 tal methods; and only suggestions, hypotheses, or guesses as to their 

 origin and nature are possible. These may serve, however, to make evi- 

 dent some of the problems they present and to indicate possibilities. Per- 

 haps it is as important to call attention in this way to some of the things 

 we do not know about development as to present established facts. More- 

 over, information concerning origin and nature of these patterns is no 

 less, perhaps in some cases even more, essential to an adequate theory of 

 development than information concerning embryonic development alone. 



APPEARANCE OF AXIATE PATTERN FROM PLANT SPORES 



Plant spores are usually unicellular but may be multinucleate without 

 cellular division of the cytoplasm. Many kinds of spores appear, with 

 different relations to the life-cycle: some apparently have nothing but 

 surface-interior pattern, others show definite axiate patterns, and in some 

 algae there seems to be little difference between spires and gametes. 



The nonmotile spores of many fungi are spherical or ovoid bodies, 

 often with no indications of axiate pattern. Germination usually consists 

 in formation of a bud from one or more regions of the spore; this elon- 

 gates, forming a mycelial filament, and gives rise to further new axes by 

 budding, often without formation of separate cells. Germination appar- 

 ently results from a local activation, as in other buds — perhaps in relation 

 to some external differential or differentials. Presence of a gradient has 

 been shown, at least in the apical regions of mycelial filaments of various 

 fungi. In certain rusts (e.g., Puccinia) germination takes place through 

 pores in the spore coat on opposite sides, so that axes arise in two opposite 



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