THE ALG.A 71 
indicate a division of labor not yet found, in which one end 
is a fixed, supporting end, the other one free. Study one or 
two cells in detail. Observe (a) cell-wall; (b) the chloro- 
plastid, a peculiar spiral affair, of which sometimes several 
are found in one cell, and within the plastid at intervals 
small rounded bodies—the pyrenoids—which are used as 
storage regions for surplus food substances; (c) the cyto- 
plasm, almost colorless, but usually appearing just within 
the cell-wall; (d) the nucleus, in the interior of the cell, and 
suspended by threads of cytoplasm.t To what part of the 
cell do the cytoplasmic threads run from the nucleus? What 
function do you think these cytoplasmic threads serve? If 
the nucleus is obscure, the addition of an iodin solution will 
usually serve to make it plain. 
How are new cells formed in the elongation of the plant? 
Draw one or two cells in detail. 
Reproduction.—Under certain conditions Spirogyra re- 
produces itself in a way quite unlike any plant yet studied. 
Two of the plant-threads come into close proximity, when 
some of their cells unite in pairs by means of outgrowths 
from the walls. Through the tube thus formed the proto- 
plasm from one cell unites with the protoplasm of the other 
cell. This united mass of protoplasm is considerably con- 
densed, and develops a heavy wall about itself. By the decay 
and breaking down of the old cell-wall this heavy-walled 
body is set free, when it usually falls to the bottom of the 
pond and passes through a resting-period, which may be 
through a drought or the winter season. When conditions 
again become favorable this resting-body grows, becoming 
a new plant similar to those which form it. This resting- 
body is a special cell, known as a spore, and is set apart for 
1 Well-prepared permanent mounts of Spirogyra and various other 
forms to be studied may be had at reasonable expense and will add 
greatly to the efficiency of the student’s work, For suggestions see 
list of addresses at close of manual. 
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