OF THE CTSTOCAEPS OF CATENELLA OPUNTIA. 71 



Vegetative Organs. 



Catenella Opuntia grows most plentifully on the protected faces 

 of rocks near high-water mark. It is anchored to the substratum 

 by numerous root-branches which spring from the under surface 

 of an irregularly-branched creeping rhizome, from whose upper 

 surface arise the branched moniJiform fronds, usually from 5 to 

 15 millim. high. The rhizomic portions are generally paler in 

 colour than the erect axes, but, save that the regular moniliform 

 structure is imperfectly or not at all developed, they do not 

 differ from the erect fronds in character. The root-ramuli are 

 simply modified branches splitting up at their apices into bundles 

 of filaments, or in some cases single filaments (one cell broad). 

 In these root-ramuli the small peripheral cells of the ordinary 

 branch are much elongated, and lie more or less parallel to each 

 other in the long axis of the ramulus. They are faintly or not 

 at all coloured. 



The creeping and erect parts of the frond do not differ in any 

 essential from each other in histological structure. The central 

 portion of the frond is composed of a loose network of elongated 

 branched hyphse, which on approaching the surface become 

 shorter, ultimately branching again and again dichotomously and 

 forming radially arranged chains of small rounded or ovoid cells. 

 Those on the surface are united by their thick cell-walls into a 

 peripheral layer, covered by a cuticle, whilst those beneath have 

 small intercellular spaces and form an inner transitional layer 

 to the retiform medulla. The cells of this intermediate layer are 

 varied in form, but longer than the cells composing the peripheral 

 layer (PI. XIV. fig. 10). The apex of every ramulus is occupied by 

 a tetrahedral apical cell. Protoplasmic continuity exists between 

 all the younger cells of the frond ; but later on this continuity- 

 seems to me to become interrupted by the growth of two plug- 

 like thickenings which are at first rings, and finally discs. Prof. 

 Schmitz, in the MS. notes which he has favoured me with, says 

 " dieser AufTassung kann ich nicht beistimmen," and refers me to 

 his published work on the subject *. I find that he expresses 

 it as his opinion that whenever a cell divides, a peculiar opening 

 is formed in the septum, by means of which the two new cells 

 remain in communication with each other so long as they are in 

 a living condition. This opening is, according to Schmitz, closed 



* M Unters. u. d. Befruct. d. Florid.," SB. Ak d. Wiss. Berlin, 1883. 



