240 MORPHOLOGY. 



occasionally find milk vessels or passages, as. for instance, in Rhus. 

 More frequently, liber-cells containing milk (in Apocynacece}, or true milk 

 vessels (as in Ficus Carica), or milk passages (as in Mammillaria qua- 

 drispina), take the place of the simple liber-cells. 



The middle cortical layer, which is properly only traversed by the 

 liber-cells and the parts by which they are represented, consists mostly 

 of roundish, very loose cellular tissue, generally containing much chloro- 

 phyll. Here and there we find it ranged in vertical rows. Individual 

 cells, or rows of cells, with crystalline accumulations, coloured juices, oils, 

 &c., or with disproportionately thickened walls, are frequently inter- 

 spersed : occasionally three or more cells, the uppermost and lowermost 

 of which, being acutely pointed, form peculiar fusiform groups, and then 

 usually contain peculiar substances (as, for instance, Pinus sylvestris). 



The external cortical layer has hitherto, as far as I know, been entirely 

 overlooked* ; it appears, nevertheless, seldom to be wholly absent, and 

 in a large number of plants, and groups of plants, it is so distinctly 

 characterised, that it quite forces itself upon one's notice. It is only in 

 a few plants that it has met with any attention, and there it has been 

 described as a liber-bundle, although it really differs very materially 

 from liber. The following are the characteristic marks of this tissue, as 

 distinctive from cortical parenchyma. The cells of this layer are always 

 vertical, elongated, very thick walled, but soft, and so far similar to 

 liber-cells ; they are, however, always applied upon one another by hori- 

 zontal walls, seldom exceeding ^y^th of an inch in length. They almost 

 invariably exhibit more or less large pores, which frequently form dis- 

 tinct, beautifully ramified canals in the thick walls ; they contain little or 

 no chlorophyll, but merely homogeneous, colourless, or sometimes reddish 

 juices, and here and there crystals. The cells are always connected 

 together by intercellular substance, and their limits therefore frequently 

 obliterated, so as to make them appear like apertures in a soft pulpy 

 mass ; when the cells are separated from each other, the intercellular 

 substance shows itself with remarkable distinctness between them, as a 

 secretion from them ( 59.). This layer is found in many plants 

 most strikingly developed, and sharply defined from cortical paren- 

 chyma, although in various modes of distribution: 1. As a per- 

 perfectly closed layer (penetrated in some cases only by small canals 

 opening into stomata), as in most of the Cacttf, Melianthus major, 

 Euphorbia splendens, Syringa vulgaris, Begonia argyrostigma, Allan' 

 thus glandulosa, Rosa, Aristolochia Sipho, Piper rugosum, Cacalia 

 Jicoides, Cotyledon coccinea. 2. Divided into many bundles, so that the 

 green cortical parenchyma reaches the epidermis between them (in which 

 case we find stomata there), as in the Chenopodeaccs, Amaranthacece, Mal- 

 vacece, SoJanacece, Umbelliferce\, Justicia, Eranthemum, &c. 3. Where it 

 may be quite distinctly recognised as a special layer, but still passing 

 quite into parenchyma at the borders, as in Carya, Pyrus Mains, Pavia, 

 Iledera, Acer, Daphne, Ptelea, Rhus, Viburnum, Cornus, Ficus, Semper- 

 vivum globiferum et laxum, Sedum pallidum, Cotyledon arborescens. 

 4. More completely merging into cortical parenchyma, and therefore less 



* Hartig maintains that he was the first to draw attention to this tissue ; as, how- 

 ever, the researches on which he grounds this claim are at present unknown to me, I 

 am unable to say with what justice. However, I attach less to being the first to observe 

 a thing, than to observing it correctly. 



f See my papers on the Physiology and Anatomy of the Cacti. 



j The so-called liber-bundles are beneath the epidermis in these five families. 



