DIV. I 



MORPHOLOGY 



43 



differ in their optical appearance and their chemical composition. 

 The secondary thickening layer is usually the most strongly developed, 

 and forms the chief part of the cell wall. The tertiary or innermost 

 layer is usually more highly refractive (Fig. 71 Ci) ; it consists as a 

 rule of cellulose. 



Cell walls which do not adjoin other cells (Figs. 40, 44) and 

 especially the external walls of the plant are, on the other hand, 

 asymmetrically constructed. In such cases thickening layers can only 

 be deposited on the side of the original 

 cell wall which faces the cavity of the 

 cell. 



2. Pitting. The cell walls which 

 separate the protoplasts will evidently 

 render difficult the passage of materials 

 from cell to cell in proportion to the 

 thickness of the wall. The life of the 

 organism could not continue without 

 such transport of material. It is there- 

 fore necessary that this should not be 

 too greatly hindered by the thickening 

 of the walls that ensures rigidity. The 

 difficulty is met by the formation of pits 

 in the walls between the protoplasts, 

 while pits are as a rule wanting in the 

 free external walls. 



The pits, which in greatly thickened FlG - ^o.-ceiis from the cortex of iris 

 walls form canals with circular (Figs. 

 38 w, 39 m) or elliptical cross section, 

 meet accurately, and would form one 

 continuous canal were it not that the unthickened primary wall 

 persists as a pit membrane (Figs. 38 t, 39 p, 40 t). The openings of 

 narrow elliptical pits into adjoining cells usually appear to cross one 

 another obliquely. 



The structure of pits may be very easily seen in the greatly thickened and 

 abundantly pitted cell Avails of the seeds of various Palms, Liliaceae, and other 

 Monocotyledons (Ornithogalum, Fig. 39). The thickening here consists of a herni- 

 cellulose which forms a reserve material in the seed, and at germination is dissolved 

 by an enzyme. The walls have a gleaming, white appearance, and are so hard that 

 such seeds, e.g. of the Palm, Phytelephas macrocarpa, are technically known and 

 employed as vegetable ivory. 



3. Connections of the Protoplasts in Tissues. The harmonious 

 co-operation of all the living parts of the body, which is such a striking 

 feature of the life of an organism as a whole, would hardly be possible 

 if the protoplasts forming the tissues were completely divided from 

 one another by the cell walls. It can in fact be shown that the 

 protoplasts of the plant are united together by extremely find 



florentina. t, Pits in the stratified cell 

 wall ; i, intercellular spaces. ( x about 

 400.) 



