16 



ORGANOGRAPHY. 



are also deposited originally in a spiral direction, and in fact 

 that all secondary deposits have a tendency to assume a 

 spiral arrangement. The great advocate of such a theory 



Fig. 32. 



Fig. 32. Pitted 

 and reticulated 

 cell. 



is Schleiden. He believes that after the cell- 

 membrane has ceased to grow by interstitial 

 deposition, an important change takes place in 

 the mode in which it is nourished, all additions 

 to it being then made in successiA-e layers upon 

 its inner surface. These layers are not however 

 deposited as continuous membranes, but as spiral 

 fibres or bands. If the cell distends after such 

 deposition, then the spires which Avere originally 

 placed close together are drawn asunder. The 

 less the cell groAvs after the deposition of the 

 fibre, the more firmly is it attached to its walls. The indi- 

 vidual spires of fibres, or particular parts of the spires, often 

 grow together. From these circumstances a very varied con- 

 figuration of the cell-Avall is produced, which may be com- 

 prised under two divisions : in the first case, Avhen the fibres 

 are clearly separable, we have fibrous cells ; and secondly, 

 Avhen the fibres are so grown together that they assume 

 the form of a membrane covered Avith little dots, we have 

 pitted cells, or, as called by Schleiden, porous cells. The ex- 

 planation thus giA^en by Schleiden of the formation of pitted 

 cells cannot be considered as by any means proved to be cor- 

 rect. Others explain such a formation, by supposing a slight 

 enlargement of the primary cell-membrane to take place after 

 the secondary layer has been deposited, by Avhich this is stretched 

 and broken into little holes or slits at A-arious points. The 

 regular distribution hoAvcver of these holes or slits, combined 

 with the circumstance that each successive layer, as it is deposi- 

 ted, corresponds exactly Avith the preceding, 

 seem clearly to indicate that such a forma- 

 tion cannot be owing to such mechanical 

 distension alone, but must be regulated by 

 some general laAv. 



In some cases, as in the Ycav (^fig. 33), in the 

 Mezereon, and in the Lime, &c., Ave find a 

 spiral fibre or fibres dcA'clopcd in addition 

 to the pits, Avhich a])pcarancc also must 

 be considered as another proof of their com- 

 mon origin. 



4. Contents of Cells. — Under this head 

 are included nearly all tlie substances Avhich 

 arc formed in the plant, or Avhich haA-^e 

 been absorbed by it from the soil. We only j)ropose at pre- 

 sent to treat of those contents of cells Avliich are more com- 

 monly found, and which have an especial importance at this 



Fig. 33. 



Fig. a3. Wood-cells 



of the Yew 



{laxtM barcata). 



After Mohl. 



