^1± DR. ALLMAN, ON 



but having made some additional observations, I have thought 

 them of sufficient interest for publication in the Microscopical 

 Journal. The first part of the paper consists of the results 

 of some observations made on Aphanizomeiwn Flos-aquce. 

 This minute alga has appeared in great abundance in the 

 large pond of the Zoological Gardens, Dublin. The best 

 account w^e possess of the plant is in an excellent paper on 

 the Nostochinea^, by Mr. Ralfs ;* but as the specimens from 

 which Mr. Ralfs's description was drawn up were not in a 

 recent state, some important points of structure have neces- 

 sarily escaped him. 



A. Flos-aquce shows itself in the form of little fusiform 

 fasciculi, of a pea-green colour (Plate III., fig, 1), which are 

 most frequently seen united to one another in larger bundles, 

 fig. 2. This union of the primary fasciculi into secondary 

 ones is not permanent, and under certain circumstances very 

 imperfectly understood ; but, in some cases, depending perhaps 

 on meteorological conditions, the secondary fasciculi become 

 broken up into primary ones, or, at least, into less compli- 

 cated bundles, and the plant, which had previously lain upon 

 the surface of the pond in an extensive stratum, becomes nearly 

 uniformly diffused through the water. A return of the former 

 conditions will again cause the union of the simpler fasciculi 

 into more complex ones, and the reaccumulation of the plant 

 in masses on the su.rface. 



The primary fasciculi are composed of straight filaments, 

 which are about 1 -3000th of an inch in diameter, and possess 

 the three kinds of cells characteristic of the Nostochineae, 

 namely, the ordinary cells, the heterocysts, and the sporangia. 



The ordinary (figs. 4, 5 a, a) cells vary much in length in 

 (liflTerent filaments, and sometimes even in the same filament, 

 and not unfrequently they present evident transverse stria?, 

 which doubtless indicate the commencement of division ; the 

 endocbrome is in the form of several oval or irregular granules 

 in each cell. Under the action of iodine the contents of the 

 cells assume a dark-brown colour, and separating from the 

 walls contract towards the centre of the cell, where they appear 

 bounded by a very definite outline (primordial utricle), fig. 6 a. 

 The entire filament appeared in some cases to be surrounded 

 by an indistinct gelatinous (?) sheath. 



When the Apkanizomenon first showed itself in the pond, 

 the heterocysts were abundant ; but no sporangia could be de- 

 tected. The heterocysts fig. 5 b, are in the form of short 

 cylinders with rounded extremities, and with bluish-green 



* On the Nostochinecr. Bv Jolin Ralfs, M.R.C.S., Ami. and Mag. of 

 Nat. Hist., May, 1850. 



