OBSERVATION AND DESCRIPTION OF PENICILLIA 35 



which the interspaces are filled by branching systems. Muller (1922), 

 working with a strain reported as PenicilUum glaucmn, attributed this 

 radial habit of growth to negative chemotropism toward its own metabolic 

 products which accumulate most rapidly toward the center of the colony. 

 Smith (1923), working with a strain of P. expansum, reported that growth 

 in length occurred only at the tips of the hyphae. Cells once formed 

 might throw out branches with new growing points but the cells themselves 

 did not increase in length during the period of observation. Such a theory 

 might account for the plane surfaces of P. expansum or P. roqueforti. 

 It cannot, however, account for the contorted, cerebriform or radiately 

 wrinkled mycelia of such species as P. sioloniferum on Czapek where the 

 progressive folding of the colony must result from a continuing lateral 

 growth of mycelial elements. 



ZONATION 



Surface growth in zones is a conspicuous feature in many species and was 

 discussed extensively by Munk as early as 1912. Biourge, in his sub- 

 genus Eupenicillium, made zonation the diagnostic character of one Sub- 

 section and assigned to his hemizonate series of that Sub-section forms 

 which were indistinctly or indefinitely zonate. It has seemed to us, from 

 an examination of Biourge's descriptions and his cultures, that this basis 

 of grouping throws together too many forms of divergent relationship. 

 Zonation (fig. 5), often very conspicuous and sometimes fairly constant for 

 certain species, is an unanalyzed growth habit occurring in different series 

 throughout the whole genus. It may better be linked with other charac- 

 ters than used as a primary basis for taxonomic separation. Zone produc- 

 tion in some species is only transiently evident in the early stages of colony 

 growth; again it appears only at the latter end of growth and in incon- 

 spicuous degrees. Some species are zonate when gro"\\Ti upon one substra- 

 tum and not upon other substrata. It has seemed best, therefore, to dis- 

 regard Biourge's major groupings based upon zonation and to use this 

 character only in the separation of members in series held together by more 

 fundamental characters. 



UUscheck (1928), follo^\dng somewhat the arguments of Munk (1912a), 

 offered the hypothesis that zones appear when colonies grow rapidly, 

 secrete enzymes and by-products of their metabolism in sufficient concen- 

 tration to reduce or suppress growth and fruit formation in those bands. 

 The mycelium then advances into fresh nutrients, resumes vigorous 

 growiih and produces an abundance of fruit, only to be depressed again by 

 the excessive by-products of this heightened activity. The extension of 

 such a colony gives the appearance of zonation. 



