410 Priestly and Irving . — The Structure of the Chloroplast 
sections of the chloroplasts, which indicated the existence of a distinct 
peripheral layer to this network. This layer contained the colouring 
matter. 
To obtain chloroplasts for observation in these cases, the leaves of the 
plant were simply crushed, and some of the green mass was mounted in its 
own sap. 
As the chloroplasts are unusually large in the case of Selaginella y 
the diameter of the largest being about 0-02 mm., an attempt was made 
to obtain sections of the chloroplast with the fresh material. The sections 
of the chloroplasts shown in Fig. i were obtained in the following way : — 
The chloroplasts" were expresssd in the juice of the plant, mixed with gum 
and glycerine, frozen, and cut to the thickness of i fx by means of a Reichert 
sliding microtome. A previous attempt to obtain sections from material 
imbedded in gelatine and frozen proved unsuccessful. The result of this 
experiment leaves no doubt in our minds, that, in these causes, the chloro- 
phyll is restricted to the outer layer of the chloroplast. This outer layer 
has a thickness varying approximately from 0-003 mm. to o-ooi mm., as seen 
in section. Clearly then the actual thickness of the layer must be less. 
The green ring thus obtained did not, even under the inch apo- 
chromatic, give any clear indications of heterogeneity of structure. 
Further evidence was obtained as a result of microtoming the properly 
fixed and imbedded material. The fixatives used were, Lang’s, Merkel’s, 
and Flemming’s (stronger solution) fluids, and the sections were afterwards 
strained on the slide by various reagents, the best results being obtained 
by the use of gentian violet, and light green. 
Material treated in this way gives evidence of a type of structure such 
as would be seen if a hollow network were cut across. 
In the case of Chlorophytum , the chloroplasts, after similar treatment, 
showed the same type of structure. A peripheral network was clearly 
indicated, even in the case of the fresh material. 
The Splitting of the Chloroplasts. 
Nageli was the first to notice the curious appearance obtained in the 
chloroplast when the usual liquid medium in which it lies is replaced by 
one of a different osmotic strength. 
Timiriazeff has since confirmed these observations, which, however, 
have not received much attention from other workers upon the subject. 
We have also repeated Nageli’s experiments, and have obtained 
confirmatory results. 
The granules, when extracted in their own sap, show no signs of such 
a crack. But upon adding water, or dilute sugar solution, whilst the 
chloroplasts still remained under observation, they invariably split, showing, 
in consequence, a fine dark or light line, which, in a very large percentage 
