CELLS WITH SPECIFIC FUNCTIONS. 



83 



he observed in transverse and longitudinal sections of the young 

 shoot or leaf of Pinus. The first stage seen is one where a small 

 column of young cells possessing granular contents is set off from 

 the surrounding cortical or mesophyll cells, and invested by a 

 ring of rather thick-walled elements, which ultimately form the 

 guard-ring. The latter soon become differentiated, whilst the 

 central cells of the internal mass break down to form the duct, 

 the remaining granular cells persisting as the endothelial layer 

 (see Fig. 63, B). This mode of origin of a resin-duct is known 

 as lysigenous origin, in contradistinction to the scMzogenous 

 method, in which the central cells of a future canal become 

 merely separated from one another 

 along the middle lamellse. It is 

 probable that the resin is formed 

 as a product of disintegration of 

 the cell-walls of the endothelium, 

 and is thus not a direct secretion 

 of the cytoplasm. 



C. Cells in which Mineral or 

 Organic Matters may sepa- 

 rate out under certain con- 

 ditions. 



The materials which separate 

 •out in these cells are not always, 

 strictly speaking, secretions, but 

 more often of the nature of excre- 

 tions, to be got rid of later on 

 by oxidative processes, removal to 

 •other parts, or other reactions in 

 the cell ; and the substances thrown 

 out of solution, such as crystals, 

 •etc., would often remain in solution 

 in the cell-sap were it not for the fact that in order to examine 

 them sections of the tissue have to be made, the mere process 

 of cutting and exposure to the air cau.sing, in many instances, 

 spontaneous crystallisation. Sometimes, however, mineral matters 

 separate out in the living cell. In this connection it is convenient 

 to examine here the following structures : — 



-Clustered Crystals 

 IN SOME OF iHB Cells of 

 A BuD-soALE or Prunus 

 lauroctrasus. 



