DIATOMS, THE JEWELS OF THE PLANT—-WORLD"* 
By ALBERT MANN 
To anyone familiar with the beautiful plants which form the sub- 
ject of this lecture it seems strange that so few people know of their 
existence, for they are abundant everywhere. The evident explana- 
tion of this, however, is the extreme minuteness of these organisms, 
most of which are wholly invisible to the naked eye. Among the 
4,000 or more species there is one, Coscinodiscus rex, a perfect 
Goliath among his brethren, which is nearly as big as the head of an 
ordinary pin; but with this exception, the larger forms can better 
be compared to the point of the pin, while many are so extremely 
small that the highest powers of the microscope are needed to dis- 
play their form and the carvings with which they are ornamented. 
The diatoms belong to the group of flowerless, aquatic plants 
known as the Algz. Where these are divided into six groups the 
Diatoms constitute one of the six; thus (1) Rhodophycez, the red 
alge; (2) Phzophycee, the brown alge; (3) Chlorophycez, the 
green alge; (4) Bacillariz, the diatoms; (5) Heterokonte, the 
yellow-green alge; (6) Cyanophycee (Myxophycez), the blue- 
green alge. Jam disposed, however, to classify them as a sub-order 
in the Order Conjugate belonging to the green alge, or Chlorophy- 
cee; and for the following reasons—(1) they have a unicellular 
thallus, (2) they have large, elaborate and symmetrically arranged 
chloroplasts, (3) they frequently produce resting-cells with thick 
walls, (4) they secrete gelatinous masses in which the individuals 
are embedded, (5) they display a double mode of multiplication, 
namely, that by fission, or division of one cell into two, and that by 
sexual reproduction by means of non-motile isogametes. The at- 
tempt to classify them with the brown alge is absurd. 
The distribution of the diatoms is practically universal. They 
occupy all waters, torrid, temperate, and arctic; fresh, salt, and 
brackish; still and running. There is hardly a brook, pond, puddle, 
lake, river, or sea on earth that is destitute of these plants, unless 
the water be so contaminated with poisonous matter as to inhibit all 
life. The largest and most elegant forms belong to the tropics, but 
1 Delivered at the U. S. National Museum, Washington, D. C., March 18, 
1905, under the auspices of the Washington Biological Society. 
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