HYDRODICTYON. 



[ 331 ] 



HYDRODICTYON. 



counties of England. The frond consists of 

 a green open network of filaments attaining 

 a length of 4 to 6" when full-grown (fig. 342), 

 composed of a vast number 

 of cylindrical tubes (cells) 

 with rounded ends, adherent 

 together at their extremities, 

 the points of junction corre- 

 sponding to the knots or in- 

 tersections of the network. 

 The individual cells attain a 

 length of 4'" or more. The 

 organization of this plant and 

 its development are exceed- 

 ingly curious, and it has 

 lately been the subject of very 

 careful investigation by Al. 

 Braun and others. The cells 

 forming the links of the net 

 have a remarkably thick cel- 

 lulose coat when full-grown, 

 which exhibits several layers, 

 especially when treated with 

 sulphuric acid (PL 38. fig. 24 

 m). Weak sulphuric acid does 

 not affect the outer layer, 

 which may be termed the 



Cuticle, while it Swells the in- of the largest size. 



ner, and throws them into 

 waves, especially the innermost ; the subse- 

 quent addition of iodine colours the inner layer 

 blue, but not the cuticle. Strong sulphuric 

 acid acts differently; it detaches the cuticle 

 at many points, while the inner layer con- 

 tracts, so that the cuticle appears blown up 

 in vesicles ; the inner layers gradually soften 

 and dissolve. 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 with iodine alone when thus par- 

 tially decomposed by natural causes. The 

 contents of the cell present several points of 

 interest connected with the phenomena of 

 cell-life, indicating a complexity in the or- 

 ganization of the internal structures not 

 formerly suspected, but which appears to 

 prevail pretty generally. 



Immediately lining the wall is a mucilagi- 

 nous layer (PL 38. fig. 24 6), which Braun 

 has shown to consist of several lamellae : 

 1 . An extremely thin, finely punctated layer, 

 coagulated and detached from the cell- 

 wall by the action of acids; this is the 

 primordial utricle of the cell. 2. The outer 

 mucilaginous layer, thicker than the primor- 

 dial 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 indi- 

 stinctly 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 

 imbedded 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 uniformly scattered. 

 Thislayer likewise contains starch-corpuscles, 

 such as occur in the green substance of the 

 Confervoids generally, causing the cell- 

 contents to exhibit a vast number of bril- 

 liant points. In imperfect cells the green 

 layer sometimes appears in patches, not 

 completely investing the surface 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 complete 

 new nets like the parent, which sets 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 opake ; lighter 

 spots appear on the inner part of the muci- 

 laginous 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 abrowner 

 tinge. The light spots already observed, the 

 centres of the nascent gonidia, exerting an 

 attraction as it were on the chlorophyll -glo- 

 bules, become severally enveloped in a layer of 

 them, and then separate from each other, so 

 that they then 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 ilium . 

 The parent cell-membrane now begins to 

 soften and swell up ; the gonidia, thus ac- 

 quiring more space, become rounded, and 

 soon present a slight tremulous oscillatory 

 movement. The cuticle of the parent-cell 

 then cracks, allowing the inner softened 

 layers to swell out ; the gonidia commence 

 an active trembling and jerking motion, not, 

 however, moving far from one spot ; after a 

 time they again come to rest, and become 

 united at certain points of their circumfe- 

 rence ; the green granules become fused into 



