478 



PHYSIOLOGY. 



of a vertical axis ; and (3) the tabular, in which there is an excess of deve- 

 lopment in the direction of the two transverse axes. 



The spheroidalforra. presents every possible transition from the sphere (Pro- 

 tococcus, figs. 513, 514, pollen of Passiftora, Hibiscus, cells of cortical paren- 

 chyma, fig. 520, &c. ), through the ellipsoidal (usual in longer or shorter forms 

 in the subepidermal parenchyma of leaves), to the fusiform or spindle-shape 

 (most abundant in the cells of wood and fibrous structure, fig. 521), and 

 the truly cylindrical, either of moderate length (cells of Confer vre, fig. 

 512, &c.), or drawn out so as to become what is termed filiform (cotton 



Fig. 523. 



Fig. 523. Section of a septum of an air-canal in the petiole of Saqittaria. 

 Fig. 524. Stellate cellular tissue rrom the petiole of Kush. Magn. 300 diam. 

 Fig. 525. Stellate hair from the petiole of Nymphcea advena. 



Magn. 300 diam. 

 rn. 

 Magn. 200 diam. 



and other cellular hairs). The spheroidal form also passes gradually, 

 especially in epidermal tissues, into the tabular form. 



Modifications. Secondary modifications of these forms arise 

 chiefly either from partial cohesion in lax tissues, from irregular 

 growth, or from pressure in densely packed tissues. 



Thus the spheroidal form becomes, in lax tissues, an irregular spheroid 

 in endless varieties (commonest of all in the parenchyma of leaves and 

 rind of succulent stems), running out by degrees into lobed" and finally 





