CHAPTER IX. 

 ARRANGEMENT OF THE PRIMARY PARENCHYMA. 



Sect. ii8. The primary parenchyma, in so far as it is not a constituent of the 

 vascular bundles, forms, as often indicated in earlier chapters, the principal contents 

 of the space enclosed by the epidermis, where it is left free by the vascular bundles. 

 Within this space it is traversed by other, non-equivalent forms of tissue, as will be 

 stated in succeeding chapters. The regions characterised in the preceding pages as 

 external cCrtex, pith, and medullary rays, as well as the leaves and foliar expansions, 

 are therefore, as regards their main mass, built up of parenchyma. 



Within the limits of this general plan, however, definite rules exist for the dis- 

 tribution and arrangement of the particular forms of parenchyma distinguished in 

 Chap. I, Sect. 3, and these rules will be here given. In doing so, attention will chiefly 

 be directed to the relatively thin-walled forms, and we shall return to the sclerbtic 

 forms in the next chapter, on account of their close anatomical and physiological 

 relations to the sclerenchyma. As the occurrence and distribution of the different 

 individual forms is chiefly determined according to the various organs of the highest 

 degree in which they occur, and to their anatomical regions as distinguished in the 

 previous chapter, it is expedient to classify the description with reference to. these 

 organs and regions, in such a manner that first the parenchymatous masses charac- 

 teristic of each will be described, and then their limiting and sheathing layers. 



Sect. 119. The parenchyma of the pith and of the bundle-cylinder of the stem 

 consists in general of cells arranged chiefly in longitudinal rows, without any very 

 remarkable anatomical peculiarities ; when young they contain products of assimila- 

 tion, and in foliage shoots frequently chlorophyll, and they either preserve this 

 condition of their contents throughout life, or, as is especially the case in Dicotyle- 

 dons, they soon dry up and die off. In the pith of many Dicotyledons, the two 

 conditions occur in different rows of cells, which then also differ with respect to the 

 form and size of their elements, as is shown by the examples of ligneous plants 

 investigated by A. Gris, which are to be cited below. From the earliest period of 

 development onwards, intercellular spaces containing air appear in the pith, which 

 sometimes persist in the form of narrow interstices, sometimes form wide lacunse, 

 while in the numerous cases of stems which become hollow in the internodes, owing 

 to their cells dying off, they are widened to form the axial air-canals described above. 

 Cf. Sects. 51, 52. 



In most stems belonging to the Dicotyledonous type, the elements of the pith, 

 as stated in Sect. 116, become narrower towards the inner boundary of the ring of 

 bundles ; together with the innermost portions of the vascular bundles, they here con- 



