HYDRODICTYON. 



[ 401 ] 



HYDRODICTYON. 



Fig. 338. 



contents becoming at once broken up into 

 active gonidia, not by successive subdivi- 

 sions (PI. 5. tig. 1). Green. Found upon 

 stones or filamentous Confervas in fresh 

 water, and consisting of elliptical sacs or 

 cells, acuminate above and below, about 

 1-500" long and 1-1200" thick. _ 2 species. 



BIBL. Al. Braun, ALy. Unicell., Leips. 

 1855, 24. 



HYDRODIC'TYON, Roth. A genus of 

 Siphouaceae (Confervoid Algae), containing 

 one species, H. utriculatum, found in fresh- 

 water pools in the midland and southern 

 counties of England. The frond or cceno- 

 bium consists of a green open network of 

 filaments attaining a length of 4 to (3" when 

 full-grown (iig. 338), composed of a vast 

 number of cylindrical tubes 

 (cells) with rounded ends, 

 adherent together at their 

 extremities, the points of 

 junction corresponding to the 

 knots or intersections of the 

 network. The individual 

 cells attain a length of 4'" or 

 more. The organization of 

 this plant and its develop- 

 ment are exceedingly curious; 

 and it has lately been the 

 subject of very careful inves- 

 tigation by Al. Braun and 

 others. The cells forming 

 the links of the net have a 

 remarkably thick cellulose 

 coat when full-grown, which 

 exhibits several layers, espe- 

 cially when treated with sul- 

 phuric acid (PI. 47. fig. 24 J). 

 Weak sulphuric acid does not 

 affect the outer cuticular A complete frond 

 layer, while it swells the 

 inner layers, and throws them size. 

 into waves, especially the in- 

 nermost ; the subsequent addition of iodine 

 colours the inner layers blue, but not the 

 cuticle. Strong sulphuric acid acts diffe- 

 rently : it detaches the cuticle at many 

 points, while the inner layers contract, so 

 that the cuticle appears blown up in vesicles ; 

 the inner layers gradually soften and dis- 

 solve. These last changes are similar to 

 what takes place at the dissolution of the 

 cell when the contents escape ; and Cohn 

 states that the membranes give the bluish 

 reaction wdth iodine alone when thus par- 

 tially decomposed by natural causes. 



Immediately lining the wall is a mucila- 

 ginous layer (PI. 47. tig. 24^?), which Braun 



Hydrodictyon 



utricuiatum. 



has shown to consist of several lamellae : 

 1. An extremely thin, finely punctate layer, 

 coagulated and detached from the cell-wall 

 by the action of acids ; this is the primor- 

 dial utricle of the cell. 2. The outer muci- 

 laginous layer, thicker than the primordial 

 utricle, but thinner than the next or third 

 layer. When separated from the first layer, 

 the outer surface appears rough and wavy ; 

 and it is connected with the third layer by 

 mucilaginous cords ; it contains indistinctly 

 defined colourless granules. 3. The inner 

 mucilaginous layer, the thickest of the three, 

 is rough on the outside and waved on the 

 inside from the projection of granules im- 

 bedded in it ; this is the only green layer, 

 appearing of a homogeneous green colour 

 (like the spiral bands of Spirogyrd) when 

 the cells are in their prime, besides which 

 it contains innumerable green granules, 

 sometimes in rows, more frequently uni- 

 formly scattered. This layer likewise con- 

 tains starch-corpuscles, such as occur com- 

 monly in the green substance of the Confer- 

 voids, causing the cell-contents to exhibit a 

 vast number of brilliant points. In imperfect 

 cells the green layer sometimes appears in 



Satches, not completely investing the sur- 

 ice of the outer mucilaginous layer ; this 

 is also common in young cells. The fluid 

 in the cavity of the cell is clear and watery. 

 The reproduction of the fronds of Hydro- 

 dictyon is effected by the conversion of the 

 contents of the individual cells into com- 

 plete new sets like the parent, which lets 

 them free by dissolution. The following is 

 a brief history of this remarkable process. 

 The first stage is the solution of the starch- 

 grains; the green layer becomes more 

 opaque ; lighter spots appear on the inner 

 part of the mucilaginous layer, excavated 

 in its substance and surrounded by the 

 chlorophyll-globules, which separate from 

 each other, forming dark boundary lines 

 round the light spots. The bright-green 

 then gives place to a browner tinge. The 

 light spots already observed (the centres of 

 the nascent gonidia), exerting an attraction 

 as it were on the chlorophyll-globules, be- 

 come severally enveloped in a layer of them, 

 and then separate from each other, so as to 

 appear like dark spots with an intervening 

 reticulation of bright lines. The dark spots 

 (gonidia) are now polygonal, mostly six- 

 sided, about the 1-2500" in diameter. The 

 parent-ceh 1 membrane next begins to soften 

 and swell up ; the gonidia, thus acquiring 

 more space, become rounded, and soon pre- 



2D 



