INTRODUCTION TO CRYPTOGAMIC BOTANY. 14-7 



division takes place in the lower parts, at a connectino- cell, 

 while, in the upj^er parts, there is merely a dislocation, and 

 sliding of the two divisions over each other. In some cases, 

 as in Scytonema, the two portions grow equally at the divided 

 ends, penetrate the outer tube together, and thus form the 

 binate ramification, which is so peculiar in that genus'; if, 

 however, the thread of endochromes protrudes without rup- 

 ture, a large loop appears at the side, instead of the twin 

 branches, and if this structure is repeated, we have a reticulate 

 frond of greater or less regularity, according as the increase 

 has taken place upon one system only, or a combination of 

 several. The greater or less tenacity of the outer tube has 

 influenced, in some measure, the characters of the vegetable, 

 but this sometimes goes much farther ; the tube itself consists 

 often of distinct coats, the number of which increases upwards. 

 These sometimes originate with such regularity as to produce 

 the beautiful striated appearance, which is so remarkable in 

 Petalonema; but in cases where there is not so much regularity, 

 the different coats are often distinctly marked, and, in some 

 instances, where the chain of endochromes extends beyond 

 the tube, the edge of the tough compound tube is lacerated, 

 and curled back, so as to present a series of frills, which 

 are complicated in consequence of the divisions of the chain 

 adhering to each other, and forming more or less regular 

 ramifications. In Desmonema Dillwynii, which has in 

 some parts of the frond, large connecting cells, and in others 

 sunply the ends of the threads attenuated and applied to 

 each other, like the ends of an ill-set bone, the outer tube is 

 sprinkled with obtuse linear bodies which, perhaps, are indi- 

 cations of roots. Branched anastomosing root-like articulated 

 threads are freely given off from the tube of Rhizonema 

 interruptum, which is, moreover, composed of cells, a fact 

 attended by great difficulties, though not without parallel in 

 the spores of certain Fungi. The cells do not seem at all con- 

 formable with the endochromes, and their formation is jDossibly 

 entirely independent of them. That the tube should sometimes 

 be marked with a spiral line, is far less surprising. Schizo- 

 gonium approaches very near to Ulva; and the species, such as 

 10* 



