36 COMPENDIUM OF GENERAL BOTANY. 



suitable. In passing it should be noted that spiral vessels often con- 

 tain a number of parallel fibres (sometimes as many as twenty); also 

 that spirals as shown at 5 (Fig. 15) are designated as sinistrorse in 

 botanical terminology (beginning at the side facing the observer 

 they extend upward from left to right). 



The tracheids of insects and the vessels of plants are formed 

 upon similar physiological principles. 



So far only the surface view of the thickened walls of secondary 

 vessels (<?, d, e) has been presented, in order to avoid confusion. 

 The view, corresponding to the outermost surface (^highest focus 

 of the microscope), does not show us the entire structure of these 

 organs. In III we will study the profile view of these vessels 

 (hence cross-sections). 



There are a number of special cases belonging to the category of 

 fibrous and linear thickenings. For the most part these will be 

 referred to in the discussion of special structures of tissues and in 

 the chapter on reproduction. Here will be mentioned only the 

 thickenings of the guard-cells of stomata and the membrane-thick- 

 enings of the dynamically active cells which cause the opening of 

 anthers. Two isolated instances may yet be mentioned as belonging 

 here : the thick pillars in palisade-cells of Cycas leaves, which very 

 probably serve as a protection against longitudinal pressure during 

 dry periods ; and the cellulose projections from the inner surface of 

 the cell-wall of the marine alga Caulerpa, which apparently serve to 

 prevent the collapse of the cell, since no septae are present. 



III. " Porous thickening " sounds almost paradoxical, yet we 

 will use this expression to designate that form of growth in thick- 

 ness which affects the entire cell-wall with the exception of very 

 small circumscribed areas. These small areas which remain thin 

 are called pores (pits). The term pore in plant anatomy, therefore, 

 does not mean an opening through the cell-wall, but an area which 

 has remained thin. 



Physiological considerations will explain in general the uses of 

 such formations (see Fig. 16). The interchange of fluids from cell 

 to cell (living cells) takes place primarily through the primordial 

 utricle ; also through the cell-wall, the dialyzing resistance of the 

 same being less in proportion to its thinness, other things being 

 equal. This latter is true of both living and dead cells. I may state 

 here that more recent investigations have demonstrated very delicate 

 plasmic connections between cells (TANGL, MOORE, GARDINER, Rus- 



