678 Transactions of the Society. 



between the two marginal lines. From this I deduce further: 

 there is no free edge of the girdle-band, such as Pfitzer has 

 described with Pinnularia, anyhow not so long as the cell is only 

 of moderate breadth ; both edges are grown to it.* 



When this connection ceases must be discovered by future 

 researches. But there can be no doubt that at some time or other 

 a process of forcible separation must take place. This separation 

 always occurs in a segment of the old marginal line. We are 

 therefore justified in stating that at that spot the new formation of 

 cellulose takes place, whilst further on towards the new line, the 

 membrane is already solidified and capable of resistance. Some 

 proof of the correctness of this view is found in the development 

 of a strong margin near this spot ; the Hues become weaker the 

 further one goes from this margin. In the ceU, shortly before divi- 

 sion, fig. 37, the tearing-ofi" of the young girdle-band has taken 

 place. This looks like the signal for a new division; at the 

 moment of tearing off, the compressed contents in the rigid cell- 

 envelope become suddenly free, at least in one direction, and can 

 hence extend, inasmuch as the girdle-bands are drawn out like 

 telescope-tubes. If these views are correct they lead us to the 

 conviction that the older outer girdle-band is a safety-sheath for the 

 inner younger girdle-band ; it prevents injuries to the latter whilst 

 partially in a non-sihcified condition; it protects with its older 

 strong portion the younger recently formed annular portion of the 

 inner band. From this may be deduced that Achnanthes is in 

 many respects similar to the growth of the ceU-envelope of 

 (Edogonium (may we say to the large marine Confervse .^).t 



We must now cast a glance again at the sections. None of 

 the uninjured (fig. 43) confirm these assertions ; one observes from 

 marginal line to marginal line a fine simple membrane, consequently 

 without doubt twofold. But only when a section is injured and 

 its substance has been slightly removed by the knife during the 

 operation of cutting is the real state of affairs brought to view. In 

 fig. 44, otherwise very similar to fig. 43, we see a portion of such 

 a section in which the young girdle-band can be traced; a de- 

 pression being nowhere observable in the space up to the old 

 marginal line, the girdle-band must have grown there. We also 

 find in such shghtly injured sections clear proof that it easily tears 

 off at the old marginal line. If I wanted to convince the reader of 



* I might mention en passant the physiological objection against the non-con- 

 nection of the two valves, that the water must find access to the inner space, 

 however narrow, and would thus come into direct contact with the protoplasm, 

 through which the latter, according to all established experience, would swell 

 and possibly effect a separation of the halves. 



t Whether the enlargement of the cell-envelope of Rhahdonema adriaticum 

 (vide Nageli and Schwendener, 17, p. 544) has its cause in a similar law seems to 

 me to require fresh investigation. 



