10 THE DIATOMACE^E OF PHILADELPHIA AND VICINITY 



OIL GLOBULES. It has been established by Pfitzer that starch and sugar, as assimila- 

 tion products, are replaced by oil in the cells of diatoms ("da bekannlich Staerke und Zucker 

 bei den Bacillariaceen nicht nachzuweisen sind ") . The oil drops are more or less numerous, 

 of various sizes, and are found in the cytoplasma, the cell-sap, and sometimes the chroma- 

 tophores. Mereschkowsky describes certain globules as elseoplasts, which he divides into 

 four kinds according to their number and position. Whether all of these are oil globules is 

 a question not yet determined. 



Other bodies, known as ' ' Buetschli granules, ' ' or volutin, and described as ' ' little blisters 

 filled with a tolerably robust refractive substance," are considered by Lauterborn to be a 

 nitrogen reserve store. They are found in the cytoplasma, or in the cell-sap, and can be 

 fixed in picric acid and stained in methylene blue. 



NOTE. For a discussion of the morphology of diatoms and a valuable re'sume' of 

 the investigations of Buetschli, Karsten, Lauterborn, Mereschkowsky, Mueller, Pfitzer, 

 Schuett, and others, the student is referred to "Der Bau der Diatomzelle," by Dr. Otto 

 Heinzerling, in "Bibliotheca Botanica," 1908. 



CELL DIVISION 



The growth of diatoms follows the usual method of cell division as described by Sachs 

 (Text Book of Botany, 2nd ed., p. 16): "The nucleus of a cell which is about to divide 

 becomes broader, assuming the form of a biconcave lens, and its nucleolus breaks up into 

 irregular granules which together with its other granular contents begin to form a nuclear 

 disc in the equatorial plane. A delicate striation is now apparent in what is becoming the 

 long axis of the nucleus, at right angles to the nuclear disc, and the characteristic nuclear 

 spindle is gradually produced. The nuclear disc splits into two halves lying side by side, 

 each of which travels to the corresponding pole of the nucleus; thus two nuclei are consti- 

 tuted which are connected by fibrillse." 



The cell-wall and the chromatophore bands divide, each nucleus passes to the centre, 

 and two new cells are formed. In the meantime, to permit of this division, the two sili- 

 ceous valves separate, the girdle bands slipping over each other, and opposite the larger or 

 enclosing valve a new valve is formed, the girdle band of which is seen later within the girdle 

 of the mother valve. Opposite the smaller valve of the original cell and adjoining the new 

 valve, another valve is formed which also produces a girdle within the girdle of the smaller 

 valve. As a result of division we have, therefore, the valves of the original, or mother cell, 

 the two new valves and four girdle bands. (PI. 40, Figs. 18 and 19.) 



In the process of division, the continual formation of new valves, enclosed in the older 

 girdle bands, will naturally cause a reduction in the size of the frustule. While this reduc- 

 tion, owing to the elasticity of the girdle, does not always occur, I believe, yet, in most 

 cases, the diameter is so reduced that a rejuvenescence of growth is required. This is caused 

 by the production of auxospores which may appear without conjugation. In this process, 

 the beginning of which, in certain species, may be noticed by the increase in the size of the 

 girdle as in reduplication, the two valves separate and within is formed a more or less spherical 

 mass about twice the size of the original frustule and which forms on its circumference two 

 large and often shapeless valves. These valves form others which assume the appearance of 

 the original valves, but larger, and proceed to grow in the usual way. The reduction in size 

 of the frustule seldom proceeds further than about half the size of the type form, so that, as 

 a general rule, it may be stated that diatoms are not often smaller than half the larger size. 



REPRODUCTION 



The process of reproduction has been observed in many cases, but the conclusions 

 reached are somewhat at variance with each other. The auxospore formation is simply a 



