170 



PART II. VEGETABLE HISTOLOGY. 



The intercellular spaces in leaves, particularly those of land 

 plants, are usually larger than those of the stem and root j those 

 which, in land plants, contain air, are usually small, except the 

 hollows of stems like the 

 grasses, many Umbellif- 

 erge, etc., while those of 

 aquatics are very large 

 and regular in form. The 

 latter are called air pas- 

 sages, and they serve not 

 only to render the organs 

 which bear them buoy- 

 ant, but doubtless the air 



which they contain is 

 serviceable in the respir- 

 atory processes in the 

 plant. The peculiar in- 

 ternal hairs called tricho- 

 blasts, which are often 

 found projecting into 

 these air passages, have 

 already been referred to. 

 They are shown in Fig. 



433- 



Intercellular secretion 

 reservoirs are common in 

 many plants, and they are 

 also either schizogenous or 



Fig. 433. 



Fig. 433 (A). -Portion of transverse section of 

 stem of Yellow Water-lily. a. trichoblast; b, inter- 

 cellular air-space. Magnified about 50 diameters. 

 (B). — Longitudinal section of stem of same plant. 

 The letters referring to same parts as in (A). Mag- 

 nified about 50 diameters. Drawing by Martin 

 Heinemann. 



Fig. 434. —Secretion reservoir from bark of 

 rhizome of Aralia nudicaulis; a, starch-bearing 

 parenchyma; i, secreting cells with finely granular 

 contents; c, intercellular space into which the secre- 

 tions are poured. Magnified about 125 diameters. 



lysigenous in their origin. They may 



