Conducting Tissue-System in Bryophyta . i 7 
these elements, which are exactly like and continuous with 
those of the central strand of the aerial stem, and which 
Haberlandt has, we think, conclusively shown to possess at 
least a chiefly water-conducting function in the aerial stem, we 
propose to adopt Potonies term hydroid . , the water-conducting 
tissue, as a whole, being called hydrom Y . The above facts 
relating to the central cylinder have all been described by 
Haberlandt in P. juniperinum (op. cit., pp. 369-70). Bastit 
fails to distinguish properly between the hydroids and stereids. 
In most cases the three-lobed central strand is clothed with 
a layer of fairly thin-walled living cells. Round each broad 
projecting lobe this layer abuts immediately on the arc of 
endodermis, and may fairly be considered as of the nature 
of a pericycle 2 . Its cells are considerably elongated, with 
1 Potoni^, Ueber d. Zusammensetzung der Leitbiindel bei d. Gefasskryptogamen. 
Jahrb. d. k. bot. Gartens zu Berlin, ii, 1883. As a term in physiological anatomy 
we think this is preferable to Haberlandt’s word hadrom, which also includes 
associated parenchyma, since it directly expresses the function of the tissue in 
question rather than the non-essential character of stoutness. Oltmanns (Ueber 
die Wasserbewegung in der Moospflanze, Strasburg, 1884) objected to Haber- 
landt’s interpretation of this tissue as mainly water- conducting, but on insuffi- 
cient grounds, which Haberlandt has sufficiently refuted. Coesfeld (Beitrage 
zur Anatomie und Physiologie der Laubmoose, Bot. Zeit., 1892, p. 153) takes 
the same view, and states that the occurrence in these cells of oil and proteid, noted 
as exceptional by previous observers (Goebel, Die Muscineen, Schenk’s Handbueh 
der Botanik, Bd. ii, p. 369, Breslau, 1887, and Haberlandt, op. cit.), is general, 
that small grains of starch also occur in considerable numbers, and that a thin 
lining of protoplasm can be demonstrated. He suggests that the cells have 
a water-storing rather than a water-conducting function. Pie produces however 
no valid criticism of Haberlandt’s experiments. Ouj own observations, so far as 
they go, certainly do not indicate the general occurrence of organized contents 
in these cells, though we have -often found in them a good deal of organic substance 
mainly of proteid nature, and in both P. commune and P. junijberinum we con- 
stantly found a number of cells with dense proteid contents in the upper part of 
the cylinder of the aerial stem close to the base of the sporogonium. We have not 
been able to plasmolyse these cells, but we have not investigated the question of 
the existence of a protoplasmic lining very carefully. It is possible that such may 
exist, but in any case no nucleus is present. We regard the importance of the 
water-conducting function of this tissue as established, but no doubt there is a good 
deal of work yet to be done on the physiological anatomy of these tissues. 
2 These pericyclic arcs are not continuous with the pericycle but with the hydrom- 
sheath of the aerial stem, owing to the way the transition takes place (see pp. 22 and 
23). It should be noted that the layer we have called pericycle in the aerial stem is 
not, like these layers, stelar conjunctive, but is differentiated from the inner cortex. 
C 
