ee BOTANICAL GAZETTE [DECEMBER 
minimum growth of fungus. Undoubtedly under field conditions 
the temperature is constantly less than 25° C., except for a short 
time during the warm afternoons. In fact, practically any night 
during the germination period (middle of April to middle of May) 
a freezing temperature may be recorded. 
Certain seeds which actually germinated or commenced to ger- 
minate had been injured in the region of the root cap or even in the 
region of the meristem of the root. This was the cause of a 
decreased vitality in the entire plant and was often the occasion 
for rapid bacterial infection. This injury was originally due to the 
Typetenid parasite in the receptacle of the flower head. Such an 
injury must be a source of constant decay to germinating seeds in 
wet ground. 
Structure 
Roor.—In the developing seedling of B. sagittata at a very early 
stage, a day or so after the seedling begins to break through the 
testa, certain cells begin to differentiate into protoxylem. These 
occur at four angles of the root section, forming a tetragon, giving 
rise to the tetrarch structure of the primary root. At first these 
spiral tubes develop singly, but may later be followed by one or 
two others centripetally at each angle of the tetragon (fig. 7). As 
might be expected from their later origin, these secondary spiral 
vessels are somewhat larger than the elementary vessels. At this 
earliest differentiation of protoxylem there are no indications of 
protophloem from procambium. Very soon, however, such differ- 
entiation begins midway and slightly centrifugal to the line joining 
the first quartet of protoxylem elements (fig. 8). The procambium 
cells in this region divide tangentially, with apparent irregularity, 
developing protophloem externally and at the same time inter- 
mediate protoxylem internally. Such growth is represented in 
figs.g and 10. These periclinal divisions continue until 4 or 5 con- 
centric rows of phloem are formed and until the xylem almost com- 
pletely envelops the axial plate. At this time the axial plate is 
still composed of undifferentiated tissue quite irregular in contour, 
strikingly similar to the stem pith of the plant. The leptome 
strands are limited externally by the undulating endodermis, con- 
