148 



LECTURE IX. 



the assimilating tissue does not differ essentially from ordinary parenchymatous 

 fundamental tissue, which may also be termed nutritive tissue in the wider 

 sense. The distinctive point— not only because the green colour of the plant is 

 caused by it, but much more on account of its fundamental significance for 

 the whole nutrition of the plant — lies in the presence of the chlorophyll-grains, 



which we may shortly distinguish 

 as green-coloured portions of 

 the protoplasm of these cells. 

 Since, as I shall show later, the 

 chlorophyll cells of the assimilat- 

 ing parenchyma, the typical forms 

 of which are found in the green 

 foliage leaves, must maintain a 

 vigorous exchange of gases for 

 the purpose of assimilation ; and 

 since, at the same time, the nu- 

 tritive water of the soil is con- 

 ducted to them, which they exhale 

 into the. air in the form of aque- 

 ous vapour"; it is intelligible why 

 the assimilating parenchyma in 

 general possesses a spongy cha- 

 racter. Its cells separate from 

 one another until nearly isolated, 

 often supporting one another only 

 at single, narrow, circumscribed 

 places, and forming numerous, 

 large intercellular spaces, which 

 usually communicate in all direc- 

 tions, and in which carbon dioxide, 

 oxygen, and aqueous vapour can 

 move from place to place. Since the 

 assimilating tissue can perform its 

 function only under the influence 

 of light, we find it always in the 

 form of thin layers immediately 

 accessible to the light. _ Thick 

 layers of chlorophyll would have 

 no purpose : since thin layers of 

 even or to 0-5 mm. thick absorb 

 the useful light rays, layers lying deeper would thus obtain no more useful rays. 

 Hence the assimilating tissue generally occurs in the form of plates, which are 

 very thin but extensive in surface, and which in ordinary thin foliage leaves are 

 covered only by an epidermis, abupdantly supplied with stomata. For the reasons 

 named, moreover, even in the very thick leaves of succulent plants and the Crassu- 

 lacese, species of Aloe, Agave, &c., only a thin lamella of green assimilating tissue is 



FIG, 157 r.— Small portion of a transverse section through the testa of the 

 seed of a Gourd. 



