52G SECRETORY AND EXCRETORY SYSTEMS 



formed in the young wood of Liquidamhar orientalis and L. styradflora 

 as an immediate consequence of mechanical injury. 



III. EXCRETORY RESERVOIRS. 



It has already been explained (p. 486) that the distinctive charac- 

 teristic of excretory, as opposed to secretory, organs consists in the fact 

 that the metabolic by-products are not liberated, but on the contrary 

 are permanently deposited within the cells. Excretory substances, in 

 fact, never exude unless the plant is injured ; they are accordingly 

 frequently made use of for protection against animal foes. 



1. Resin- anel oil-sacs. 2m 



Sacs containing resins or ethereal oils are found in the paren- 

 chymatous tissues in the Zingiberaceae, Piperaceae, Lauraceae, 

 Magnoliaceae, Aristolochiaceae, Canellaceae, and in many Euphor- 

 biaceae ; further, in the genera Acorus, Aloe, Rheum, Lysimachia, etc. 

 They occur singly, or are aggregated in groups, or, more rarely, 

 arranged in definite series. Most frequently they are relatively large, 

 and isodiametric and rounded in shape. The contents consist mainly 

 or exclusively of the excretion, which in many Zingiberaceae and Piper- 

 aceae, and in Acorus Calamus, takes the form of a colourless or light 

 yellow ethereal oil, but which is often more deeply coloured. The 

 seriated sacs associated with the vascular bundles in certain species of 

 Aloe, are filled with a dark-coloured sometimes with a colourless 

 liquid, in which small drops of resin are often suspended. The " chryso- 

 phane-sacs " of the Rhubarb-root may contain a homogeneous orange- 

 coloured sap, or a colourless liquid with bright red drops suspended in 

 it. Occasionally {e.g. Aloe and Acorus, according to Johow) even 

 fujly developed resin- or oil-sacs retain a peripheral protoplasmic 

 layer and a nucleus. The cell-walls of such cells are always thin and 

 devoid of ornamentation. Zacharias states that they are very fre- 

 quently suberised, or at any rate provided with a suberin-lamella. 



Many excretions are certainly deposited in cell-cavities. Berthold, 

 on the other hand, has observed, in a wide range of families, that 

 excreted oil drops may be enclosed in a sac- or balloon-like internal 

 protuberance of the cell-wall. The stalk of this vesicular process is 

 cutinised ; after treatment with sulphuric acid it persists in the shape 

 of a little cup attached by its base to the wall, whereas the delicate 

 membrane of the vesicle itself is dissolved. The author has not him- 

 self followed out the development of these organs in detail, but has 

 confined his attention to the adult structure of the oil-cells of Laurus 

 nobilis and Asarum curopaeum. His observations entirely confirm 



