372 Wisconsin Academy of Sciences, Arts, and Letters. 
amount of slimy cytoplasm; other hyphae contain a large amount 
of slimy cytoplasm and nuclear structures filled with reserve 
food (fig. 16). 
Mycelium of Phycomyces forty days old was placed on moist 
nutrient agar and after sixteen hours many hyphae had grown out 
from the old mycelium. Preparations made from sections of this 
material showed that nuclear structures of the type described 
above for aged mycelium were abundant in the new hyphae in the 
substratum. Some had lost completely the reserve food of the 
vacuoles and the nuclei were in a state of active division. Most of 
the nuclear structures were in a transitory condition in which the 
vacuolar portions had swollen considerably; the reserve food was 
partially dissolved and some of the nuclei were beginning division. 
Figure 15a shows several nuclei in a very dense cytoplasm which 
apparently has been formed from the reserve substance of the 
vacuolar portion. Figure 15b shows a nuclear structure in which 
most of the reserve food has been dissolved and the vacuole is quite 
clear; one of the nuclei is in progress of division and a second 
nucleus is apparently unchanged. 
The foregoing nuclear behavior fits in well with the growth habits 
of these plants. The plant grows rapidly with a favorable water 
supply and during the growing period the nuclei divide rapidly 
and are distributed into the newly formed portions of the plant. 
As soon as a decrease in available water takes place or, at least, 
after a certain minimum is reached, growth stops and at the same 
time nuclear division ceases. As this change occurs most of the 
nuclei are at a certain stage in division. The nuclear vacuoles 
cease to expand but apparently proceed to absorb food reserve 
from the surrounding cytoplasm, giving rise to nuclear structures, 
filled with reserve food, as has been described above in old dried- 
out mycelium. When such mycelium is again supplied with wa¬ 
ter, new hyphae are observed growing out from the aged hyphae. 
The nuclei and associated structures resume activity, giving up the 
food reserve and dividing as previously described. 
Discussion 
In the above report it has been difficult to use the term ^‘nucleus” 
and convey the full idea that the writer holds as to its form and 
structure. It is evident that the fundamental unit of structure 
composing the nuclei is a small, homogeneous mass that stains 
