5QO MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



frustule is considered to present its ' front ' view when its line of 

 meeting is turned towards the eye, as in fig. 453, B, C ; whilst its 

 ' side ' view is seen when the centre of either valve is directly 

 beneath the eye (A). Although the two valves meet along the line 

 of junction in those newly formed frustules which have been just 

 produced by binary subdivision (as shown in fig. 445, A, e), yet, as 

 soon as they begin to undergo any increase, the valves separate from 

 one another ; and by the silicification of the cell-membrane thus left 

 exposed a pair of hoops is formed, each of which is attached by one 

 edge to the adjacent valve, while the other edge is free. 1 As will 

 be presently explained, one of the valves is always older than the 

 other ; and the hoop of the older valve partly encloses that of the 

 younger, just as the cover of a pill-box surrounds the upper part of 

 the box itself. 2 As the newly formed cell increases in length, 

 separating the valves from one another, both hoops increase in 

 breadth by additions to their free edges, and the outer hoop slides 

 off the inner one, until there is often but a very small ' overlap.' 

 As growth and binary division are continually going on when the 

 frustules are in a healthy vigorous condition, it is rare to find a 

 specimen in which the valves are not in some degree separated by 

 the interposition of the hoops. 



The impermeability of the silicified casing seems to render neces- 

 sary the existence of special apertures through which the surrounding 

 water may come into communication with the contents of the cell. 

 Some have believed that they have seen such apertures along the 

 so-called ' line of suture ' of the disc-shaped diatoms, and at the extre- 

 mities only of the elongated forms. Ehrenberg, followed by KUtzing, 

 has interpreted as apertures or ostioles the central and terminal 

 nodules of the Navicalecv, Cymbellew, and similar forms; but this 

 view is more generally regarded as incorrect. We have, in fact, no 

 positive demonstration of the existence of special apertures communi- 

 cating between the outside and the inside of the cell ; and we are com- 

 pelled to have recourse, on this point, to hypothesis. It is, however, 

 certain that the diatom-cell is always composed of at least two valves, 

 between which the possibility of such a, communication must 

 necessarily be admitted, or at least the existence of endosinotic and 

 exosmotic currents in the liquids. In the encysted forms we have 

 ascertained also the existence of an interval between the two rings, 

 although it may be very minute ; while Navicula has been some- 

 times seen with the valves actually separated. 



1 [This refers to those diatoms in which the process of binary subdivision is 

 possible ; but this, as will be seen presently, is not the case in many genera. ED.] 



- This was long since pointed out by Dr. Wallich in his important memoir on the 

 'Development and Structure of the Diatom- valve ' (Transact, of Microsc. Soc. n.s. 

 vol. viii. 1860, p. 129) ; but his observation seems not to have attracted the notice of 

 diatomists, until in 1877 he called attention to it in a more explicit manner (Monthly 

 Microsc. Journ. vol. xvii. p. 61). The correctness of his statement has been coii- 

 iirmed by the distinguished American diatomist, Prof. W. Hamilton Smith ; but as it 

 has been called in question by Mr. J. D. Cox (American Journal of Microscopy, 

 vol. iii. 1878, p. 100), who asserts that in Isthmia there are three hoops two 

 attached to the two valves, and the third overlapping them both at their line of 

 junction the Author has himself made a very careful examination of a large series 

 of specimens of Isthmia and Biddu-lphia, the result of which has fully satisfied him 

 of the correctness of Dr. Wallich's original description. 



