Notes . 
275 
Hofmanni, Kiitz., Symploca thermal is, Kiitz., Gloeocapsa caldariorum , 
Rabh., &c.) may have been originally introduced into our parts with 
greenhouse plants ; since reliable observations on the Algae occurring 
in any given district do not date back very far, whereas the cultiva- 
tion of exotic plants in hot-houses is an old practice. Many of the 
blue-green . Algae (e. g. Scytonema cinereum , Menegh., Symphyosiphon 
Hofmanni , Kiitz., &c.) are so absolutely characteristic for every 
moist hot-house, that it seems plausible that in our parts they first 
originated here, and only later on escaped to other (more natural) 
localities. I have also observed these characteristic species in many 
of the hot-houses of the gardens at Glasnevin, near Dublin. 
The hot-house flora is practically equally developed during the 
whole year, the conditions under which it exists remaining uniform. 
The flora outside, however, shows quite a different character in the 
winter and in the summer, and attains its maximum development 
in August and September; in the winter only the hardy genera of 
Algae ( Vancheria, Oedogonium, Cladophora , Rhizocloniuni) are present, 
whilst the smaller forms (Protococcoideae, &c.) are absent. Desmids, 
not very common even in the summer \ are quite absent, as also most 
of the other Conjugates, Spirogyra crassa , Kiitz., being the only species 
that can be met with all the year round. This species usually has 
a very well-developed sheath, often as much as one-fifth of the 
diameter of the cell in thickness 2 , and the hardiness of the species 
is probably due to its presence. Desmidieae first appear in March, 
and species of Scenedesmus and Pediastrum at the beginning of April 
or a little before 3 . Even in the tanks in the warmer houses (e. g. 
1 Very few Desmids were also observed in the Plankton of the Thames ; analysis 
of the river-water shows that a considerable percentage of calcium carbonate 
is present (cp. Algological Notes, III : Preliminary Report on the Phyto- 
plankton of the Thames ; Annals of Botany, vol. xvi, 1902, p. 581). I should like 
at this spot to mention that since the publication of my note I have received from 
Professor G. S. West a paper by his father and himself (A Contribution to the 
Fresh-water Algae of the North of Ireland, Trans. Royal Irish Acad., vol. 
xxxii, sect. B, pt. i, August, 1902) which contains an account of the Plankton 
Algae of Lough Neagh during the months May, 1900, and August, 1901. In the 
same paper reference is made to a publication of Borge’s : ‘ Siisswasser-Plankton 
aus der Insel Mull,’ in Algologiska Notiser, 4; Botaniska Notiser, 1897. 
2 A somewhat similar sheath is described for Sp. lubrica by Braun ; cp. Verjiin- 
gung, 1851, p. 261. 
3 Most of these unicellular or few-celled forms probably only occur when the 
conditions of temperatures, and especially of illumination, begin to be favourable 
(cp. Zacharias, Uber die Ursache der Verschiedenheit des Winterplanktons in 
T Q, 
