204 



THE GUIDE TO NATURE. 



donk, who has suggested the pres- 

 ence of the external fluid layer, whose 

 motions result in the movements of 

 the diatom, says that he has artistically 

 stained this protoplasmic coating, and 

 has, after its death, actually seen it curl 

 up and become separated from the body 

 •of the cell. 



The structure of these remarkable 



in which the valves are not in some de- 

 gree separated by the interposition of the 

 hoop." 



The contents of these diatom boxes 

 are an exceedingly important nucleus, 

 the colorless protoplasm, and the yellow- 

 ish-brown coloring matter. Although 

 a diatom is a plant, a green diatom does 

 not exist. If the microscopist, there- 



Fig. 7. 



Fig. 9. 



DIATOM^. 

 Selected to show variety of form and of markings. 



Fig- 5- 



plants is much like that of a pill-box, to 

 which it has often been compared. The 

 entire cell, which is referred to by mi- 

 croscopists as the frustule, consists of 

 two layers that he calls the valves, and 

 which imperfectly correspond to the up- 

 per and the lower surfaces of the box. 

 Delicately connected with each valve is a 

 band of silica called the hoop, and rough- 

 ly compared to the sides of the pill-box 

 which are attached to the lid and to the 

 body. We may then form a mental 

 picture of a diatom as a usually flat- 

 tened case of silica, the hoop of the valve 

 on one side fitting loosely into the hoot) 

 of the op'x isite valve, mucn as the side 

 of the lid slips over the side of the box. 

 It is known that "one of tne valves is 

 always older than the other: and the 

 hocp of the older valve partly encloses 

 that of the younger. As the newly 

 formed cell increases in length, separat- 

 ing the valves from one another, both 

 hoops increase hi breadth by additions 

 to their free edges, and the outer hoop 

 flides off the inner one. until there is 

 often but a small 'overlap.' As growth 

 and binary division are continually going 

 on, . . it is rare to find a specimen 



fore, finds under his instrument, a motile 

 green object, he may he sure it is not a 

 diatom. Color and movement are not 

 distinguishing characters of microscopic 

 animals, but, other things being equal, 

 they may become distinguishing features 

 of a diatom. 



Reproduction is accomplished by two, 

 some microscopists say by three distinct 

 methods, and is in each instance com- 

 plex. The one most frequently observed, 

 and perhaps the simplest, takes place by 

 the division, into two parts, of the pro- 

 toplasm within the frustule. Each of 

 these parts retains one of the old valves 

 on its outer surface, and each of the two 

 inner surfaces then secretes a new valve. 

 two new individuals thus being formed, 

 each with one parent valve and one 

 young valve, so that every diatom frus- 

 tule is usually partly youthful and partly 

 aged, a condition much like that obtain- 

 able by building a new house under an 

 old roof. 



