16 



ANATOMY AND PHYSIOLOGY OF 



Ftcj 20. 



Cells of the leaf of San&eviera 

 gumeensis 



tions ia the flesh, of pears and quinces, the horny albumen of Phy- 

 tehphas, of many Palms (fig, 14^), and of the Rubiacese. These 

 smaller orifices in the secondary membrane are denominated pits, 

 the cells themselves pitted cells. The numerous transitions from 



this form of cell into the form of those hav- 

 ing a net-work of narrow fibres, and from 

 these into the spiral-fibrous cells, furnish 

 the evidence that the fibres are not, as 

 earlier phytotomists believed, to be consi- 

 dered as a peculiarly organized elemen- 

 tary portion, but that they are nothing 

 else but narrow sections of the secondary 

 membrane lying between elongated pits ; 

 that between fibre and membrane there 

 exists a distinction in form, but none in 

 essential nature. 



The distribution of the pits upon the 

 cell is usually altogether irregular, espe- 

 cially upon the horizontal transverse walls 

 of the parenchymatous cells. On the other 

 hand, it is common, and especially in elon- 

 gated cells, for the pits upon the lateral 

 walls of the cells so far to exhibit regu- 

 larity in their position, that they stand more or less exactly in the 

 direction of a spiral, and are frequently drawn out lengthways in 

 this direction (fig, 21), so that they appear as short 

 slits. Sometimes also a certain rule may be met 

 with in reference to the places on which pits exist 

 or are deficient. Thus in the wood-cells of most 

 Coniferae they are found on the side-walls, turned 

 towards the meduLary rays ; thus in loosely con- 

 nected parenchymatous cells they not unfrequently 

 occur 0^ the flattened parts of L walls byVhicl 

 the cells are coherent together, while they are ab- 

 sent from the surfaces which bound the inter-cel- 

 lular passages, as occurs frequently in the cortical 

 cells of Dicotyledons ; or, if they occur on the inter- 

 cellular passages, they differ in size and form from 

 those situated on the side- walls of the cells, e. g., 

 in CycaSy in the wings of the seeds of Swletenia. 

 The pits are moreover usually wanting to the 

 outer walls of the epidermal cells, but they may also occur here ; 

 as, for example, on the leaves of Cyca^'. 



The pits of one cell are most intimately connected in regard to 

 form and position with those of the contiguous cell; and it is a 

 general law, that when two pitted cells are coherent together, the 

 pits of the two cells lie exactly opposite to each other ; so that in 

 very thick walled cells the cavities of the two cells are only sepa- 



Fzg 21. 





c^: 



Wood-cell of Ginkgo 

 Mloha, 



