in Blechnum and Osmunda . 37 
of either iodine or osmic acid. The secretion is produced 
mainly at the expense of the endoplasm and of the substance 
of the plastids. As we have remarked before, the remains of 
the ectoplasm and nucleus may always be recognised, and they 
are usually left behind after the escape of the secretion (Figs. 
27 and 29). The changes which occur in the drops themselves 
are of great interest. At their first formation the young drops 
are watery and by no means well defined. By the use of 
osmic acid it can be demonstrated that at this stage they 
contain no tannin. They shortly become denser and more 
highly refractive, and tannin makes its appearance equally 
distributed throughout their structure. Osmic acid then gives 
very striking results, for the separate drops are blackened by 
the reagent while the protoplasmic reticulum takes practically 
no stain, and thus the contrast between the two is sharp and 
well defined. And now in the drops themselves a delicate 
reticulation may be observed, which finally gives way to the 
appearance of numerous minute and brightly shining droplets, 
all separate and distinct, disseminated throughout the sub- 
stance of the drop, just as the drops themselves are disseminated 
through the substance of the secretory protoplasm (Figs. 10 
and 13). The droplets do not contain tannin, and osmic acid 
stains the ground substance only. We have thus been able to 
trace a wonderful series of changes, which must from a 
chemical point of view be of a most interesting character, but 
which at present we cannot with certainty explain. Methylene- 
blue confirms the results obtained with osmic acid. This 
substance, as Klebs 1 has shown, is capable of forming with 
tannin a definite compound : and we ourselves have observed 
that the staining produced by the dye in structures containing 
tannin is one which is both distinct and easy to be recognised, 
especially when viewed through yellow light, such as that pro- 
duced by an argand burner. The coloration, namely, is a 
somewhat dull purple blue, not unlike that produced by 
haematoxylin, and stands out in marked contrast to the usual 
1 Klebs in Untersuch.. a. d. bot. Institut z. Tubingen, 1886. 
