446 PHYSICAL GEOGRAPHY. 



and in the life history of one of these we shall be able to study the sim- 

 plest expression of cell growth. Dr. Carpenter, whose valuable treatise 

 on the microscope is to be recommended to all intending to use that 

 instrument, has epitomized what is known on this subject so well that I 

 cannot do better than to give it in his own words. He says, 



The life-history of one of these unicellular plants, in its most simple form, can 

 scarcely be better exemplified than in the Palmoglcea macrococca (Kiitzing) one of 

 those humble kinds of vegetation which spreads itself as a green slime over damp 

 stones, walls, &c. When this slime is examined with the microscope, it is found to 

 consist of a multitude of green cells (Fig. A) , each surrounded by a gelatinous envel- 

 ope ; the cell, which does not seem to have any distinct membranous wall, is filled 

 with granular particles of a green color; and a nucleus, or more solid aggregation, 

 which appears to be the centre of the vital activity of the cell, may sometimes be 

 distinguished through the midst of these. When treated with tincture of iodine, 

 however, the green contents of the cell are turned to a brownish hue, and a dark 

 brown nucleus is distinctly shown. Other cells are seen (B), which are considerably 

 elongated, some of them beginning to present a sort of hour-glass contraction across 

 the middle ; in these is commencing that curious multiplication by binary subdivison, 

 which is the ordinary mode of increase throughout the vegetable kingdom ; and when 

 a cell in this condition is treated with tincture of iodine, the nucleus is seen to be 

 undergoing the like elongation and constriction. A more advanced state of the pro- 

 cess of subdivision is seen at C, in which the constriction has proceeded to the extent 

 of completely cutting off the two halves of the cell, as well as of the nucleus (I) , from 

 each other, though they still remain in mutual contact ; but in a yet later stage they 

 are found detached from each other (D) , though still included within the same gelati- 

 nous envelope. Each new cell then begins to secrete its own gelatinous envelope, so 

 that, by its intervention, the two are usually soon separated from one another (E). 

 Sometimes, however, this is not the case, the process of subdivision being quickly 

 repeated before there is time for the production of the gelatinous envelope, so that a 

 series of cells (F) hanging on one to another is produced. 



Now the diatom aceae grow in a manner precisely similar to that just 

 described as taking place in Palmogl&a. This subdivision of the cell, so 

 that the new individuals are formed from one in the diatomaceae, results 

 in the production of a series of frustules almost identical in all particulars 

 with the original individual. In most cases, if not in all, the new indi- 

 viduals differ from that from which they sprung in two marked respects : 

 First, they each possess one old and one new valve ; and, secondly, the 

 new valve is smaller than the old one, so that the two valves of all dia- 

 toms differ somewhat in dimensions, although alike in other respects. 



