

POSITION OF THE DIUTYOTACE^), 



465 



















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radial rows of cells (figs. 2, 3), reminding one of the mode of proce- 

 dure of the extrafascicular cambium of the stem oiDracana and 

 other arboreous Liliaceae. Interruption of the cambial zone occurs 

 at the diametrically opposite scars due to the atrophy of the thallus 

 lamina. The occurrence of secondary thickening of this nature in 

 a seaweed and in one of the Dictyotaceans, more especially from a 

 systematic point of view, of considerable interest, only two other 

 cases of the kind being known. It is well known that secondary 

 thickening very much as in JDictyopteris, but with a deeper- 

 seated meristematic layer, takes place in the stalk of the Lamina- 

 riaceae. Eeinke, in 187G, showed that in the stalk of Fucus vesi- 

 culosa * secondary thickening by the activity of a cortical cambial 

 layer takes place, twenty, or more, tangential layers being thus 

 added ; to him also belongs the credit of first finding secondary 

 thickening in Bictyopteris f, but he failed to observe the special 

 cambial layer, and did not enter into the secondary thickening 

 m full detail. Strange to say, there is no reference, in any 

 ubsequent observations, to Reinke's on Fucus vesiculosus % or 

 on Dictyopteris. Seeing the stalk thicker than the midrib, my 

 impression was, before making any sections, that possibly the 

 thickening was due to the interpenetration of hypha-like pro- 

 longations of the lower cells of the midrib, a mode of thick- 

 euing found in the basal part of the thallus of Ulva, Bangia, 

 Porpkyra, and in the Fucacece. Growth in thickness, as it occurs 

 m Bidyopteris §, is a well-marked Phaeophycean character, not 

 to be met with in the Floridea. 



s 



















Repboduction. 





* 













*> 







Th 



A. Vegetative (or asexual). — Tetraspores 



je presence of asexual reproductive bodies in the form of 

 cruciate tetraspores is considered a strong Floridean affinity. 

 ne Dictyotacea are not alone amongst FhceopTiycets in the pos- 

 session of such bodies. Keiuke, at the beginning of this year 



Reinke, Jabrb. f. wiss. Bot. x. 1876, p. 333. 

 T Bemke, "Entwicklungsg. Unters. lib. d. Dictyotaceen d. S. v. Neapel," in 



°t\ ACt Ac ' W-'Carol. vol. xl., 1878. 



* Sower and Vines, in 'Practical Botany,' pt ii. p. 51, repeat Reinke's 

 ob8 ervations. 



§ Judging from the specimens in the Herbarium of the Natural History 

 tbT"™' l belieTe man y species of Zonaria will, on examination, show similar 



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