MANN | DIATOMS, THE JEWELS OF THE PLANT-WORLD 53 
deeper parts of the sea. No actual test has been made of the ocean 
depth at which diatoms can flourish, but the limit is probably some- 
thing below 100 fathoms. Specimens of diatoms are, it is true, ob- 
tained from all depths, even from the abysses of 6,000 fathoms or 
more; but in such cases they are invariably the dead and empty 
frustules of plants that have been transported there by surface cur- 
rents. There are a few diatoms partly or wholly destitute of chloro- 
phyl and which therefore live a saprophytic life. Such is Nitgschia 
putrida, a colorless form, and Bacillaria paradoxa another Nitzschia, 
which is only partly saprophytic and therefore not wholly colorless. 
The food-product of assimilation can never be utilized by the 
individual plant to any great extent; for being encased in an in- 
TG or 1 
flexible silica box its chance of growth is restricted to a very slight 
increase in breadth by the slipping apart of the upper and lower 
valves or lids: in other words, each diatom is formed at its own 
maximum length. Most of the reserve food is therefore utilized in 
the multiplication of individuals. This takes place in two curious 
ways. The first and common method is the asexual one of fission, or 
the separation of a single plant into two along a median dividing 
line. Although this is the usual method of multiplication in unicellu- 
lar plants as well as in single cells of multicellular plants, the process 
in the diatoms is peculiar in taking place, not transversely, that is 
along the short axis, as is done in the bacteria, the other alge, etc., 
but longitudinally from end to end. This peculiarity is the origin of 
the name “diatom,” from déa toyéw, to cut through. An examination 
of Figs. 10 and 11 will make this process clear. It is easy to see that 
