488 



PHYSIOLOGY. 



with the long woody fibres in the liber of Dicotyledons. In 

 these cells, which that author calls " latticed " or " clathrate " cells, 

 the membrane which forms the diaphragm closing large pits is 

 marked with an excessively delicate network, apparently formed 

 of fibres applied upon the primary membrane, and generally per- 

 forated. This occurs not only in the pits of the side-walls, but in 

 those which are found on the septa between cells standing one 

 above another, and which constitute ihe sieve disks of the Germans, 

 the said disks being covered on one side with a thick hyaline struc- 

 tureless perforated plate, not coloured blue by iodine, and whose 

 nature and functions are at present unknown. 



Spiral Deposits. The " fibrous " secondary layers may present 

 the form of a single spiral band, running from one end of the 

 cell to the other, and with the turns of the spire quite close or 

 more or less distant (fig. 538) ; or the spiral band may be double, 



Fig. 540. 



Fig. 538. 



Fig. 539. 



Fig. 538. Cell from the sporangium of Equiseium arvense. Magn. 2nO diam. 



Fig. 539. Cells from the sporangium of Marchantia polymorpJM. Magn. 250 diam. 



Fig. 540. Cells from the leaf of Sanseciera yuineensis. Magn. 400 diam. 



triple, or even consist of six or more parallel bands. Very often 

 these spiral secondary deposits are sufficiently elastic to allo\v of 

 their being stretched out, the comparatively thin primary membrane 

 to which they adhere giving way at the interstices. 



In the cells of the coat of the seed of Cottomia, the primary membrane 

 becomes, during the ripening of the seed, converted into a substance 

 which softens and swells up in water ; so that when this structure is 

 wetted, the spiral fibre springs out, opening its coils widely like a wire 

 spring. 



