^62 Mr. G. H. K. Thwaites on Tetraspores in Alga. 



the utricular cells of the spore-sac. The primordial utricle is 

 however completely isolated in the lower cell, i. e. the immediate 

 elongation of the nucleaiy membrane itself, the contents of which 

 do not consist as before of chlorophylle, but of cytoblastema. 

 When acted upon by nitric acid, it frequently contracts so much, 

 as to appear torn into large band-like fragments, which become 

 somewhat spirally twisted (fig. 25). Its membrane is not per- 

 fectly smooth but finely granular. This does not occur so much 

 in the lowest segments of the stem. It is remarkable, that when 

 treated with nitric acid, which contracts it considerably, it ex- 

 hibits various-sized conical prominences. Hence it appears some- 

 what angular or wavy. The small projections appear generally 

 to pass into minute depressions on the axial membrane. At a 

 subsequent period a secondary membrane is formed between it 

 and the axis. 



From what has been stated, it appears that the stem consists of 

 an epidermoidal membrane, subsequently also of a secondary one, 

 the primordial utricle and the cell-contents. The above epider- 

 moidal membrane, which is the direct elongation of the nucleary 

 membrane, continues to grow with the plant , and in such a manner 

 that the plant remains in it as in a bag. Kiitzing calls this mem- 

 brane the peridermis, and considers it as identical with the cuticle 

 of Brongniart, which covers the true epidermis of more highly 

 organized plants *. There can however be no question here of 

 a true epidermis, nor indeed in any of the Algae. 



[To be continued.] 



XXXV. — On the Occurrence of Tetraspores in Alga. 

 By G. H. K. Thwaites, Esq. 



To the Editors of the Annals of Natural History. 



Gentlemen, 2 Kingsdown Parade, Bristol, March 19, 1846. 



In the last December Number of your valuable Journal is an 

 extract from a letter presented by M. Montague to the French 

 Academy on the subject of an interesting Alga belonging to the 

 Zygnematay and discovered by M. Durieu in Algiers, in which 

 the fruit consists of four distinct spores in each sporangium. 



The Kev. M. J. Berkeley obligingly favoured me with a sight 

 of an authentic specimen of this species, in which the character 

 was very obvious. 



On examining, a few days ago, some spores of Mesocarpus sea- 

 laris, Hassall, I thought I could detect in them indications of a 

 quaternary division, and I sent specimens to Mr. Berkeley for 



* Kiitzing, /. c. p. 86. 



