. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . each with its raphe-canal (>•). 2, trans-verse section of wing, showing raphe-canal and outer cleft. 3, longitudinal section of , wing viewed from the edge, qk, cross-canals; zw, thin intercalary pieces of wing; Ik, raphe-canal (or plasma-canal); sp, cleft of raphe. From Oltmanns (after Lauterborn). The somewhat complex system of clefts and canals described aboveis concerned with streaming movements of the cytoplasm, the latter being
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . each with its raphe-canal (>•). 2, trans-verse section of wing, showing raphe-canal and outer cleft. 3, longitudinal section of , wing viewed from the edge, qk, cross-canals; zw, thin intercalary pieces of wing; Ik, raphe-canal (or plasma-canal); sp, cleft of raphe. From Oltmanns (after Lauterborn). The somewhat complex system of clefts and canals described aboveis concerned with streaming movements of the cytoplasm, the latter beingbrought into direct contact with the surrounding medium by means ofthe outer fissure of the raphe. This is stated by O. Mliller to be the directcause of the curious movements of those diatoms which possess a true raphe.(Consult page 102.) The raphe of other pennate diatoms is essentially different. In Surirellathe lateral margins of each valve are produced into wing-like expansions,of which there are therefore four to each cell. Near the free edge of eachwing is a fine raphe-canal, with a longitudinal fissure extending its whole 94 BacillarieaB. length and placing it in communication with the exterior, while it isconnected with the interior of the cell by cross-canals (fig. 66). The freeedge of each wing is thus somewhat thickened to accommodate the raphe-canal, and its flat portion consists of alternating cross-canals (fig. 66 qk) andthin intercalary pieces (fig. 66 zw). In Nitzscliia there is a raphe-canal with a longitudinal fissure similarto that of Surirella, extending along the whole length of the keel of eachvalve. In some diatoms there is a narrow, hyaline, axial area without a central nodule, either median or submedian in is not a true raphe, since there is no cleftin the valve, and it is known as a pseudoraphe. Itmay be present on both valves, or, as in Cocconeis,Achnanthes (fig. 67), and Rhoicosphenia, limited toone valve only, the other valve p
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