'HE VEGETABLE CELLf 



^o 



It is scarcely \yorfcli mentiomng tliat; Meyen {'^ Fhysiohglep i. io) 

 set lip the tlieory tliat not only tlie secondary layers, but also the primary 

 membrane was composed of distinct spii'al fibres grown together. Ho 

 was led to this opinion principally by the cells containing a very fine 

 spiral fibre, of a Stells gathered by laini in Manilla, the structure of which 

 he completely misapprehended, since he imagined that the fibres formed 

 the primary membrane, while they belonged to the secondary. 



In conclusion, it may be remarked that Schleiden's hypothesis ("-5e^ 

 trage^'i. 187), that in the formation of the secondary layers there exist at 

 first, at least, two spiral bands, one corresponding to the ascending current, 

 the other to the descending current of the mucilaginous formative sub- 

 stance, the two extremities coalescing at the ends of the coil, and that in 

 most cases these become blended together at a very early period, is simply 

 to be banished into the region of dreams. 



The opinion which formerly prevailed widely, and which Link (" FML 

 Botanr 1837, i. 177) still defends, that the pits of the scalariform ducts 

 and pitted vessels are the remnants of the fibres of spiral vessels broken 

 up into fragments, requnes no further refutation. Holes in a membrane 

 can scarcely be considered as elevations. 



Ohserv, 4. In the preceding I have spoken of cells and vessels as clearly 

 separated organs, because in most plants the fully-developed cell differs in 

 a marked manner from the fiilly-developed vessel ; but it must not be 

 forgotten that transitional structures occur. One form, the porous cells, 

 has already been mentioned ; these come near to the vessels in the large 

 open pores, by which they communicate with each other, but they are dis- 

 tinguished from those by the fact, that they form a parenchymatous tissue 

 in the manner of cells, lie upon the surface of 

 oi'ffans, and, in part, in jSphagnwn (-^s^. 33, B), 

 open eVen to the external dr/yMe thf vascular 

 utricles are always combined into tubes, whicli 

 ran among the cells in the interior of plants. 

 Another intermediate structure occurs in the vas^ 

 cular Cryptogamia, particularly in the Lycopodia 

 and Ferns, as well as in the Coniferse and Cyca- 

 dejB. In these plants we meet with the peculiar 

 condition that the wood is not composed of a 

 mixture of elongated cells and vessels, but of ele^ 

 mentary organs of one kind, which resemble pro™ 

 senchymatous cells in their form, and vessels in 

 the structure of their walls, and give evidence of 

 their near relation to the latter, in the fact that 

 the prolongations of the vascular bundles of the 

 stem%ntering bto the leaves, contaii perfectly 

 developed vessels ; as also ta the fact, that in the 

 stems of Coniferse and Cycadese, the innermost 

 elementary organs, bordering on the pith are per- 

 fect spiral vessels, and that in B^ilhedra particular 

 ■wood-cells become united into perfect pitted ducts. 



Observ. 5, Perhaps it is not altogether superfluous, ia reference to the 

 terminology of the pitted cells and vessels, to remark that since the struc- 

 tui^e of the pits (tdpfel) and their distinction from actual holes have been 



m'\^. 



Porous cell fiinuslicd with 

 am*-ular fibros from tlie leat'of 



