CHEMICAL STIMULATION OF GROWTH OF ASPERGILLUS NIGER 369 
object for the illustration of the different kinds of growth: cell division, 
increase in dry weight, and increase in volume; not to mention differentia- 
tion. At the apex the cell divides, increases in volume here being due to 
increases in the organic compounds of the cell, and hence in dry weight due 
to the imbibition and assimilation of foods and water. Farther back, in 
what is commonly referred to as the zone of elongation, the rapid increase 
in size of the cells appears in the main to be due to absorption of water and 
its accumulation in vacuoles, without necessarily an increase in the dry 
weight of protoplasm present. Lastly we note that certain cells undergo 
a still further modification of form in the zone of differentiation. The dis- 
tinction between the different kinds of growth is not always sharply marked. 
Neither are the primary forces making for the intake of water and foods 
by the cell determined as yet with any certainty. An increase in proto- 
plasmic volume may be attributed to an increased imbibition of water, 
just as an increase in size of the vacuoles is attributed to osmotic phenomena. 
The absorption of foods by the organism results in the formation of colloids, 
which, however, would not necessarily increase the imbibitional capacity 
of the protoplasm. Those materials which accumulate as soluble bodies 
in the vacuoles and lead to increase in size of course play a very important 
part in the elongation of the so-called growing region of the root. In A. 
niger the thickening of the cell walls (differentiation) also probably con- 
tributes largely to the end results of the growth acceleration in the presence 
of zinc and iron. It is certainly of interest that in animal tissues, in tissues 
of cacti, and in the hyphae of A. niger, increase in acidity within certain 
limits {i.e., optimum pn or acidity) may lead to increased absorption of 
water, or of nutrients that are further elaborated, or of both. 
It is clear that increased acidity and the specific effect of certain heavy 
metals are not the sole means by which growth is accelerated and spore 
formation retarded or suppressed in the case of "chemical stimulation" 
of A. niger. Such compounds as chloroform, ether, etc., which act as 
"stimulants," do not, as far as we at present know, increase the acidity of 
the nutrient solution. No purpose is served by speculations attempting 
to suggest uniformity where none may exist. 
My studies were made under the direction of Professors W. G. Marquette 
and R. A. Harper. To both I am greatly indebted for advice, aid, and 
encouragement. 
Summary 
I. Increased acidity of the Pfeffer nutrient solution within a certain 
range results in the exhibition in Aspergillus niger cultures of growth 
"stimulation" like, but less in amount than, that obtained by addition of 
salts of certain heavy metals. The appearance of the cultures duplicates 
exactly, in decrease in sporulation and in formation of opaque hyphae 
resulting in thick, white, wrinkled membranes, that of the classic zinc 
cultures. 
