642 PATTERNS AND PROBLEMS OF DEVELOPMENT 



less orderly character in which the main axis and branches consist of the 

 secretion and the diatoms are in groups at the tips. Here, also, some degree 

 of integration and apparently some degree of dominance are involved as 

 regards growth and division of the different groups ; but these forms are 

 less regular in growth than the pseudothalli, and nothing is known con- 

 cerning possible physiological differences in the diatom groups of different 

 branches. Obviously, the patterns of these supercellular integrations can- 

 not be inherent in the individual diatoms but must originate in reactions 

 to physiological conditions determined by cells as environment of other 

 cells and also to the external environment. 



DEVELOPMENT OF MULTICELLULAR AXIATE PATTERN IN CERTAIN FUNGI 



In the mushrooms and toadstools among the basidiomycetes and in 

 some forms among the ascomycetes — the morel and related species — the 

 vegetative stage consists of indefinitely branching hyphae, each consisting 

 of a single series of cells; these hyphae usually form a subterranean felt- 

 work, the mycelium. Each hypha and branch represents an axis with 

 growing tip, and a gradient has been demonstrated in young, growing 

 hyphae of certain fungi; but hyphae of basidiomycetes have not been ex- 

 amined. Development of the fruiting body or sporophore, the mushroom, 

 toadstool, or morel, begins as an aggregation of hyphae at some region of 

 the mycelium. By more or less parallel upward growth of hyphae the 

 aggregation rises above ground as a small rounded structure. This elon- 

 gates by further, more or less definitely directed growth; and the form of 

 stipe and pileus or cap, inclosed in a loose feltwork of hyphae, the volva, 

 becomes distinguishable in it. In forming the pileus the hyphae change 

 direction of growth; and in development of "gills" or other parts from 

 which the basidia (modified ends of hyphae) arise and form spores, further 

 changes in direction of growth of hyphae in orderly and definite relation 

 to pattern of the whole are involved.'^ In some of the basidiomycetes 

 there is considerable further regional differentiation, and pigmentation 

 of the upper surface of the pileus is characteristic of some forms. Even 

 after development of pileus and stipe, every cell of Coprinus is capable, 

 on return to nutritive medium, of forming vegetative mycelium, and from 

 this, new sporophores. In certain forms evidence of dominance and physi- 

 ological isolation appears in the dependence of branching on inhibition of 

 the apical region (Brefeld). 



The origin of axiate pattern in the mushroom or toadstool differs from 



'* See, e.g., Brefeld, 1876; Magnus, 1906; P. Kohler, 1907. 



