SUPPLEMEN7~ 



Some marine plants use much iodine, or soda, or calcium, each plant 

 selecting from the thousands of gallons of water flowing over it the 

 special mineral it requires. 



These manufactured organic substances of course serve primarily 

 for the growth of the plant, that is, for the extension of protoplasm 

 and the envelopes produced by it, but surplus food is stored, often in 

 the form of starch as in the water net. The starch can be identified 

 by putting iodine on the colorless plants taken from the alcohol, and 

 comparing the color of the grains with color of wheat flour similarly 

 treated. The iodine of commerce will answer. 



Beyond comparing the size of the smallest nets with the largest 

 mature cells, the children will have to take the story of reproduction 

 on authority, but it can be made an interesting one. Develop from 

 the children the fact that the little protoplasts join hands and form 

 nets for safety. They are not so easily swept away or swallowed by 

 little animals. The statement that the stronger method of 

 reproduction by resting spores occurs as danger approaches is based 

 on laboratory experiments, in which the two kinds of reproduction 

 are induced by simulating favorable and unfavorable natural condi- 

 tions. The spore formed by the union of the two tiny protoplasts does 

 not grow directly into a new net. It first divides into several pecu- 

 liarly shaped cells which may ultimately become nets. 



Instead of water net, the children may find Algae that consist 

 entirely of threads or filaments. They may be rough to the touch 

 and branched, as No. 5, Fig. 4, which represents Cladophora, or water 

 flannel. No. 6 is an outline drawing of part of the same under the 

 microscope. The individual cells closely resemble those of water net. 

 Or the children may find pond-scum or water silk consisting of slimy 

 unbranched filaments. Nos. 3 and 4 represent two kinds of water 

 silk, Spirogyra and Zygnema, under the microscope. As the chloro- 

 plasts of these plants are spiral bands or more or less star-like masses, 

 these plants are particularly attractive under the microscope. All of 

 these filaments lengthen by a process called cell division. From 

 about the middle of the cylindrical cell wall a ring of cellulose, as the 

 substance comprising the cell wall is called, pushes its way toward 

 the center of the cell until it becomes a circular partition dividing the 

 former cell into two cells which soon attain full growth. In Clado- 

 phora branches push out at first like little swellings. All of these 

 filamentous Algae reproduce by fragments, just as higher plants are 

 propagated by slips or cuttings. Cladophora reproduces also by 

 zoospores, that is, the many protoplasts into which a mature cell may 

 divide, escape and swim about for a time before growing into new 



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