EXTRACTS FROM FOREIGN PERIODICALS. 
151 
coloured in the same manner. A thickness not its own is therefore sometimes 
attributed to the substance. The same precaution must be observed in all the 
woods of Dicotyledons. In Coniferte, for instance, the lines of separation be¬ 
tween the substance and the cells are much less cut off than those between the 
internal and external layers of these cells. It is, therefore, easy to make a 
mistake. 
In order clearly to observe this matter in the cellular tissue of wood, the hard 
tropical woods, or our Box-tree, should be selected. It is much more difficult to 
detect it in the majority of our indigenous trees, because the cells are there 
extremely small, and closely connected. 
It is well characterised in the elongated cellular tissue of the bark and 
petioles of the greater part of phanerogamous plants, as the Elder (Sambucus 
nigra.') 
In parenchyma, or rounded cellular tissue, the cellular matter is so small in 
quantity that it generally eludes observation. Hence the existence of passages 
which are but empty spaces caused by the scarcity of this substance. It is, 
however, still obvious in tough leaves, as the Laurel (Laurus nobilis). The 
facility with which the cellules can be separated sufficiently proves its existence 
in all rounded cellular tissue, even in cases where its extreme tenuity and 
transparence prevents its being distinctly seen. 
The albumen of many Monocotyledons contains it, whether between the cells 
or at the surface, and it covers them with a homogeneous layer (as in Liliam 
martagon). 
The external membrane which M. Brongniart has detached from the cuticle 
by maceration, is but a homogeneous layer of the same substance, which fills 
not only the intervals of the cells of the cuticle, but also covers them on the 
outside with this pellicle. 
Lastly, the external envelop of grains of pollen is almost always composed 
entirely of intercellular substance. 
These and other observations illustrate the important part which the substance 
under consideration performs in the vegetable kingdom. Its study, carried still 
further, will one day lead to interesting results on many physiological points. 
The theory of the ascent of juices by the intercellular passages is already rendered 
little probable, since these passages are only exceptions to the normal state of 
the plant. 
This substance is semi-fluid, sometimes hard and solid, but never fibrous, 
transparent, perfectly homogeneous, and almost always very hygroscopic. Its 
chemical properties appear to be analogous to those of the cellular tissue which 
it envelops, and vary, consequently, according to the species and the organs in 
which it is observed. But M. Mohl has not yet distinctly described its chemical 
nature and physical properties. 
