THE VEGETABLE CELL. 



17 



rated from each other, in the canals of the pits, by the primaiy 

 walls, which form a very thin partition (figs. 11, 14, 15). This 

 dependence of the structure of one cell upon that of its neighbours, 

 becomes the more piominent the more the reticulated foimation 

 prevails in the secondary membranes, and it disappeais in propor- 

 tion as the spkal structure becomes more distinctly evident. 

 Therefore where the pits are scattered irregularly they correspond 

 accurately in form and position ; where they are arranged in a 

 spiral direction, and present the appearance of short elliptical slits, 

 they correspond in position but no longer in form, since being situ- 

 ated obliquely in the opposite direction, they cross and only cor- 

 respond at their central portion (fig. 21). Finallyj when the pits 

 are extended into long spiral slits, surrounding the cell, the rela- 

 tion to the contiguous cells has altogether disappeared. 



In thick walled cells the pits usually form cylindrical canals, 

 which, however, frequently open into the cavity of the cell by a 

 funnel-shaped opening at their inner extremity; and sometimes 

 the outer blind end is somewhat enlarged. Not unfrequently two 

 or more pit-canals unite into one common passage, opening into 

 the cavity of the cell (fig. 1 2). 



In many cases the piimary walls of two contiguous cells sepa- 

 rate from each other at the spots where the pits lie and leave a 

 lenticular cavity between them, which has a rather larger circum- 

 ference than the 



pit itself (fig. 22) ^'^ ^^ 



and then ap- 

 pears like a ring 

 surrounding the 

 pit (fig. 23). I am 

 only acquainted 

 with this struc- 

 ture in elon- 

 gated cells ; it is 

 most distinct in 

 the wood -cells 

 of the Coniferae 

 and^ Cycade^, 

 but it occurs in 

 the wood -cells 



of many Dicotyledonous trees. These cavities are not yet existent 

 in very young cells, but they are found before the dej^osition of 

 the secondary membranes, and the formation of pits arising out of 

 this. Schleiden's assertion that these cavities arise from the secre- 

 tion of a bubble of air between the previously blended cell- walls is 

 incorrect ; they are filled with sap in the young condition of the cella 



In isolated, but very rare, cases, the piimary membrane which 

 is stretched across the pits as a partition, becomes absorbed after 

 the completion of the development, whereby the pitted cells be- 



Fifi 2^ 



Transverse section througrh a 

 pit (a) of Fz'ma PiTiea. 



The pit of Pmu» Pima seen 

 m fate, a, canalof ihupifc, &, 

 border. 



