. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . ight years later (Lutkemtiller, 02) published animportant account of the general structure of the cell-wall in Desmids. Afurther paper by the same author emphasized the importance of the structureof the cell-wall in formulating any sound scheme of classification of Desmids(vide Liitkemuller, 05). DesmidiacesB 357 In a few Desmids—those belonging to the Spirotsenieae among SaccodermDesmids—the cell-wall consists of a single layer of cellulose,
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . ight years later (Lutkemtiller, 02) published animportant account of the general structure of the cell-wall in Desmids. Afurther paper by the same author emphasized the importance of the structureof the cell-wall in formulating any sound scheme of classification of Desmids(vide Liitkemuller, 05). DesmidiacesB 357 In a few Desmids—those belonging to the Spirotsenieae among SaccodermDesmids—the cell-wall consists of a single layer of cellulose, all in one piece,and of a homogeneous structure. In the remaining Saccoderm Desmids, ofthe Gonatozygae, the cell-wall is also quite continuous in one piece, but thereis a differentiated outer layer. The great majority of Desmids belong to thesub-family Placodermoe, in which the cell-wall is composed of two welldifferentiated layers of varying thickness. Micro-chemical tests show thatthe inner layer is practically structureless and consists of pure cellulose. Theouter layer is stronger and thicker, consisting of a ground substance of cellulose. Fig. 221. Various species of Stauraxtrum. A, St. pelagicum W. & G. S. West, x 520; B, De Not., x 520; C, St. monticulosum Breb. var. pulchrum W. & G. S. West, x 520;D, St. pungens Breb., x520; E, St. saltans Joshua, x 520; F, St. cyclacanthum W. & Gr. , x 520. a, front view of cell; b, vertical or end view ; d, basal view of semicell. impregnated with various other substances, amongst which compounds of ironhave been demonstrated. It is in some species of Closterium and Peniumthat the iron compounds are most prominent, and this is presumably directlyconcerned with the yellow-brown colour of the cell-wall so often seen in thesegenera. The pores pass through both layers of the cell-wall and in the outer layereach pore is surrounded by a cylindrical tube-like structure which does notconsist of cellulose. To the
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