276 TISSUE ELEMENTS OF SEED PLANTS 



their neighbours, and act as special water bladders 

 (Fig, 43, F). And the leaves of certain plants growing 

 in places where they are specially liable on occasion to 

 lose water quicker than it can be supplied have a 

 many layered thin-walled epidermis, and this massive 

 water tissue loses water to the mesophyll and partially 

 collapses, bellows fashion, under such conditions. Thick 

 leaves often possess a central water tissue. 



Thick-Walled Cells — Pits. — Many of the hving cells of 

 a plant thicken their walls considerably after the cell 

 has grown to its definitive size. This thickening of the 

 wall is often general or uniform over the whole wall 

 surface, and may be carried so far that the cell cavity is 

 greatly reduced in volume (Fig. 44, C, D, E). Successive 

 layers of cellulose are laid down by the cytoplasm, and 

 the layering (stratification) of the wall can often be 

 clearly seen under the microscope (Fig. 44, B, C). 

 This appearance is supposed to depend upon the smaller 

 or greater amount of water contained in successive 

 layers of wall, making the cellulose more or less solid 

 in consistency. 



The continuity of the layers is, however, generally 

 interrupted at certain spots, where the original wall, 

 formed at cell division (middle lamella), is not added to. 

 These thin places in the wall are called pits. Pits are 

 always formed opposite to one another in adjacent 

 cells of a thick- walled tissue (Fig. 43, E, p), so that 

 the cell cavities are only separated by the thickness 

 of the middle lamella {pit membrane). A pit is very 

 commonly circular in section, so that it appears as a 

 lighter circular area when the wall is seen in surface view 

 under the microscope. The walls of a pit are commonly 

 perpendicular to the middle lamella, so that the pit is 

 of equal diameter from top to bottom (" simple pits "). 



