PROTOPLASMIC CONTINUITY IN THE FUCACE.E. 101 



the space encloHed by the ring, and through this the protophism is 

 coutiuuous, as through a sieve plate, by a number of dehcate 

 threads. This is perhaps the commonest form of continuity, and 

 bears tlie closest possible resemblance to that met with in the 

 sieve-tubes of higher plants. (PL 255, Fig 5.) 



(in.) The third type agrees with the second, except that 

 continuity is effected by a thin and delicate ribbon of protoplasm, 

 which passes through a narrow slit in the diaphragm. This form 

 is not abundant, and appears to be an intermediate form between 

 the preceding and the next to be described. (PI. 255, Fig. 6.) 



(iv.) In the fourth type the diaphragm is complete and 

 impervious, save at the centre, where is an extremely minute pore, 

 through which a single delicate strand of protoplasm maintains the 

 continuity. (PL 255, Fig. 7.) 



Besides the cases in which continuity is more or less evident, 

 there are numerous others, in which the protoplasm still comes well 

 up to the ends of the cells, and so obscures the nature of the con- 

 nection between the contiguous protoplasts. This is worthy of 

 note, as it shows the tenacity with which the protoplasm adheres to 

 the end-plates, even after the roughest treatment, and though not 

 demonstrative of continuity, is quite consistent with it and easily 

 explicable by it. 



A few cases, again, are met with, in which the ends are clear 

 and the protoplasm has shrunk more or less in the longitudinal 

 direction. In some of these there are clear evidences of a 

 previously existing continuity in the fimbriated appearance of the 

 ends of the protoplasmic mass. 



"With respect to the delicate diaphragm met with at the ends of 

 the cells, the fact should be noted that it does not swell up under 

 the action of reagents, nor does it stain like the rest of the walls. 

 The same may be said of the annular thickening which encloses it. 

 The various degrees of perforation exhibited by the diaphragm are 

 doubtless indications of successive stages of development, the final 

 term of which may be an imperforate diaphragm and a complete 

 interruption of continuity. This, however, seems only to occur 

 when the life of the cell comes to a close and a barrier is needed 

 between the living and the dead parts of the tissue. 



Fucus. 



Among the various forms of Fucus which have been sub- 

 mitted to examination, with a view to determining the presence 

 or absence of continuity, the chief are Fucus vesiculosus and F. 

 serratus. 



The flattened, dichotomously-branched thallus of these plants 

 has a minute structure, which very closely resembles that of Asco- 

 phylluni nodosum. In transverse and longitudinal sections, the 

 epidermis, the cortex, and the central tissue, are readily dis- 

 tinguishable, and both in appearance and histological composition 

 are extremely similar to the corresponding tissues of the plant 

 named. The midrib, a feature not present in AscoplujUum, is 

 formed by a greater development of the filamentous tissue of the 

 central part of the frond. 



