the Sieve-tubes in Laminar ieae . 107 
are quite structureless when examined unstained, or in corallin- 
soda, Hoffmann’s blue, or Russow’s reagent. With hydric 
sulphate, as they swell up, a well-defined stratification may be 
made out, the lines of stratification being parallel to the free 
surface of the callus-plate. A similar appearance may be 
made out in chlorzinc-iodine. Fully formed callus-plates are 
shown in Figs. 15, 1 6, and 17. Figs. 15 and 16 are from M. 
pyrifera , collected ninety years ago. In Fig. 1 5 depressions 
are seen which are continued some distance through the 
callus-plate towards the sieve-plate. This, however, none of 
them reach. These lines are all that represent the original 
perforations of the sieve-plate which have become obliterated 
by the callus. In Fig. 16 less trace even than this is seen of 
the perforations. As a rule, however, the ends of the callus- 
plates have a number of conical depressions, as in Fig. 17 \M, 
planicaulis\ Into these continuations of the contracted con- 
tents of the tube are produced — ending of course blindly. In 
some cases in which the contents ( Schlauchkopfe ) have been 
pulled away from the callus-plate, in making the preparation, 
it can be seen that on its surface (previously in contact with 
the callus) are a number of elevations, which correspond to 
the depressions in the callus. Out of more than two dozen 
specimens of Macrocystis collected at various periods I have 
not met with a single instance in which callus-plates are absent 
from the older true sieve-tubes. 
Before entering on the development of the callus-plate I 
will speak briefly of the contents of the sieve-tubes. In study- 
ing this, herbarium-material is of little value. As a rule the 
contents are seen as a retracted sac with ends widened over 
the sieve-plates ( Schlauchkopfe ). This sac is granular and 
stains light brown in iodine. Sometimes I have found what 
seemed to be drops of mucilage in it, attached at various 
points to its outer layer, or even as small aggregations in the 
region of the sieve- or callus-plate. 
In only two cases have I found the contents not contracted 
but consisting of apparently a coagulated mucilage enclosed 
in a thin membrane— the c primordial utricle.’ This mucilage 
