176 PHOTO-MICROGRAPHS. 
moulding power which causes these plant hairs, 
arising from the free surface of the leaves of differ- 
ent species of plants, to assume such varied forms ? 
And how is it that this moulding power always 
produces little stars upon the leaf of one species 
—e.g., Deutza scabra —and simple unbranched 
hairs upon another—e. g., Brassica? This is a 
fundamental question in biology, and one that is 
only vaguely answered by science in the state- 
ment that it is in accordance with the laws of 
heredity. The case is of the same nature, al- 
though vastly more complicated in the outcome, 
as among the unicellular organisms, the simple 
life-history of which has already been detailed. 
One little mass of protoplasm enclosed in a cellu- 
lar envelope grows by appropriating to itself 
the necessary material from the nutritive me- 
dium in which it is placed, and multiplies by 
binary division, producing only little spheres 
(Micrococci), which, however, present noticeable 
differences as to dimensions. Another similar 
mass has an elongated form (Bacilli) which is 
preserved in its progeny; but these also present 
slight variations as to length and diameter. A 
third species presents a spiral form, which is pre- 
served from generation to generation. In the 
interior of these rods or spiral filaments, spheri- 
cal masses of protoplasm (spores) are formed 
which cannot be distinguished one from another 
by the highest powers of the microscope, but 
which are nevertheless potentially different, for one 
in growing produces a rod, and the other a spiral 
filament. 
