SECONDARY DEPOSITS. [ 571 



] 



SECONDARY DEPOSITS. 



the thick cell-wall to the primary membrane 

 (see Pitted Structures) (PI. 38. fig. 23). 



In the other case, the secondary deposits 

 are more sparing in quantity, and are applied 

 over lines forming a definite pattern upon 

 the primary meml)rane, in which a spiral 

 course in the direction of the long axis of 

 the cell is more or less evident ; infinite 

 modifications of this type occur, which are 

 treated under the head of Spiral Struc- 

 tures (PI. 39. figs. 7> 9). 

 ^ In certain less common cases we find the 

 earlier secondary layers exhibiting the pitted 

 character, while others later formed produce 

 spiral-fibrous thickening, as in Taxus, the 

 lime and other cases (see Pitted Struc- 

 tures) (PL 39. figs. 4, 13, \db). 



The last-mentioned cases point to a rela- 

 tion between the spiral-fibrous and the 

 pitted layers, which appears really to exist, 

 for in a great number of cases it is possible 

 to distinguish a spiral structm'e in the mem- 

 branous layers of pitted cells or even of cells 

 where the layers of thickening merely exhibit 

 the laminated structure without any pits or 

 fibrous markings. Thus, in the hber-cells 

 of the A])ocynaceae (PL 39. fig. 30), the 

 thickened walls appear under a low pow'er 

 homogeneous, while under sufficient magni- 

 fication, and especially by the help of acids, 

 w^e may detect an evident though delicate 

 spiral structure. The action of acids reveals 

 a similar spiral arrangement of the consti- 

 tuent molecules, in the cotton hair (PL 21. 

 fig. 1), and in most hber-cells (PL 21. figs. 2, 

 6, 25), in many wood-cells, as of Finns, &c. 

 The membranes forming the sheaths of many 

 of the Oscillatoriacese (PL 4. figs. 13 c?, e, 

 15) exhibit a spiral-fibrous structm'e when 

 undergoing dissolution, and an analogous 

 condition may be detected by the help of 

 reagents pretty generally in the cell-walls 

 of the tubular Confervse. All these pheno- 

 mena seem to indicate a fundamental iden- 

 tity in secondai-y layers of all kinds, to which 

 we direct attention under Spiral Struc- 

 tures, but it is convenient in practice to 

 keep the Pitted and the Spiral-fibrous 

 structures distinct. 



The mode of formation of the secondary 

 deposits is not clearly known at present ; 

 some imagine them to be precipitated from 

 the cell-sap upon the walls ; others, and ap- 

 parently with more reason, believe that they 

 are attributable to the agency of the pri- 

 mordial utricle, continuing its action 

 after the formation of the primary mem- 

 brane. Criiger goes so far as to consider 



the spiral markings, &c. as dependent on 

 the RoTATiON-currents of the protoplasm. 

 These points require further investigation. 

 There can be little doubt of the mistaken 

 character of Trecul's recent view, which re- 

 gards the spiral and other fibrous thickenings 

 as folds of the primary w all thrown inwards. 



The secondary deposits appear to be 

 always composed of some modification of 

 cellulose. Mohl has investigated this point 

 very thoroughly, and w^e have followed him 

 over much of the ground. The cellulose, 

 however, loses its distinctive character with 

 age, either by infiltration with foreign mat- 

 ters, or by a slight chemical metamorphosis, 

 so that old secondary layers do not readily 

 become blue when sulphuric acid and iodine 

 are applied ; but as a general rule the cel- 

 lulose reaction may be obtained by using a 

 preUminary treatment. All internal struc- 

 tures, such as wood-cells, liber-cells, stones 

 of fruits, &c., should be boiled in nitric 

 acid, washed, dried, and tincture of iodine 

 applied j then if again dried and wetted with 

 water they turn blue; external structures, 

 such as epidermal cells, cork and the like, 

 require a similar boihng, but with solution 

 of potash. 



Secondary deposits present a considerable 

 difference in then consistence and degree of 

 development in different cases. In most 

 wood- and liber-cells they are abundant in 

 quantity, in some cases almost filling up the 

 cavity (PL 38. fig. 27) ; here they are hard, 

 and appear to be in that state of the cellu- 

 lose compound which may be distinguished 

 as lignine. The same condition prevails in 

 the stones of fruits, bony shells, the " grit " 

 of pears, &c. ; and the less abundant second- 

 ary substance of spiral-fibrous tissues ap- 

 pears to be in the same state. The secondary 

 layers of parenchymatous cells are usually 

 rather soft and elastic, and often turn blue 

 with sulphuric acid and iodine alone ; those 

 of the coUenchymatous tissue beneath the 

 epidermis of many herbaceous plants, such 

 as the Chenopodiaceae, &c., are abundant in 

 quantity, but of somewhat cartilaginous tex- 

 ture. Those of the larger Algae, and of the 

 thallus of the larger Lichens, approach to 

 the same condition, while the fleshy and 

 horny albumen of many seeds contains 

 abundant deposits of analogous character 

 (PL 38. figs. 21-23); in the latter the com- 

 position is sometimes of amyloid, approach- 

 ing starch, stained blue by iodine alone, and 

 more or less soluble in dilute sulphuric acid. 

 The secondary layer of epidermis and corky 



