INTERCELLULAR SECRETORY RESERVOIRS. %0'] 



quickly in the air without change of colour. This must be a peculiar mixture : water, 

 alcohol, ether, benzine, and alkalies do not alter it on the whole, though each reagent 

 may dissolve a small quantity of it. When burnt it leaves behind a very small residue of 

 ash. I have not obtained a clear idea of the mode of origin of the passages of the 

 Mamillarias. 



The crystal-containing passages of the Opuntias, which may attain a width of 5™'", 

 are apparently of lysigenetic origin ; in the mass of mucilage the cells, from the dis- 

 organisation of which they arise, are still partially recognisable. 



Some details concerning the secretory cavities remain to be added, and statements 

 may also be looked for regarding their occurrence and arrangement. 



(rt) The Myrtaceae, judging from observations on numerous species of Eucalyptus, 

 Melaleuca, Callistemon, Eugenia, and Myrtus, are generally provided with oil-cauities. 

 In horizontal leaves these are particularly numerous on the upper surface, though not 

 confined to it, and their epithelial layer is contiguous with the epidermis, in which 

 those cells which touch the epithelium differ from the rest in form and size. In Myrtus 

 communis, for instance, two semicircular epidermal cells are contiguous with the wall of 

 the cavity at the upper side of the leaf: these cells are distinguished from the rest by 

 their lateral walls not being undulated, and by their being only half as high. In the outer 

 cortex of the branches, according to investigations on species of Eucalyptus, they are 

 separated by some few layers of parenchyma from the epidermis. They have an 

 approximately spherical form : the larger ones may be seen with the naked eye as 

 transparent points, others are smaller, e.g. in the leaves of Eugenia australis. The cavity, 

 which is filled with mixtures of oil and resin, is limited by a continuous epithelial layer 

 composed of tabular cells. According to Frank ^ the cavities in the leaf of Myrtus 

 communis are of schizogenetic origin. A round, thin-walled cell, lying beneath the 

 epidermis, divides successively into 8 octants ; these separate at their central point of 

 contact, so as to form directly an intercellular space filled with oil, and this gradually 

 assumes the form of the spherical cavity ; the original eight epithelial cells meanwhile 

 stretching tangentially, becoming flattened, and occasionally dividing. This description 

 is contradicted by INIartinet's short statement, according to which the oil-cavities of the 

 Myrtaceae arise like those of Citrus in a lysigenetic manner, a view which, though I have 

 not observed their development myself, I must consider to be correct, from the 

 agreement of the mature cavities with those of the Rutaceae. 



[b) The presence of oil-cavities is a general and characteristic phenomenon in the 

 members of the group Rutaceae in the sense of Bentham and Hooker, /. e. the families or 

 divisions of the Rutaceae, Diosmeae, Boronieae, Zanthoxyleae, Flindersieae, Toddalieae 

 (Skimmia), Aurantiaceae, Amyrideae^. The Simarubcie and Zygophylleae are excluded 

 from this group, and have no oil-cavities. 



The distribution of the organs in question, and their relation to parenchyma and 

 epidermis, is, as far as investigated, the same as that in the Myrtaceae. In the stem of 

 Dictamnus and Correa alba they lie directly under the epidermis, in the leaves of 

 species of Agathosma and Diosma especially or exclusively on the under side of the 

 leaf — relations which are found also in the Myrtaceae. They also correspond to these in 

 average form and size. Their origin is in all cases lysigenetic. Even Frank's drawing 

 for Ptelea trifoliata does not contradict this, though according to his description of the 

 process of development their origin is schizogenetic. Rauter ' gives a very exact history 

 of development of the oil-cavities on the upper surface of the leaf of Dictamnus (Fig. 86). 

 A cavity {A) originates from two cells, one an epidermal cell, the other a cell of the 

 hypodermal parenchyma. The first divides successively into four cells, placed crosswise 



Beitr. p. 125. 



Compare Engler, /. c. on Amyris ; also Van Tieghenn, /. c. p. 1 73. 



Trichomgebilde, &c. /. (". p. 21. 



