374 CHLOROPHYLL- GRANULES AND THE SUN'S RAYS. 



leaves, in which a distinct separation between the palisade and spongy parenchyma 

 is completed, always show many more chlorophyll-granules in the former than in 

 the latter. Careful countings have shown that the palisade cells usually contain 

 three or four times — occasionally even six times — as many chlorophyll-granules as 

 the adjoining cells of the spongy parenchyma. When the chlorophyll-granules in a 

 cell are so many that the whole inner wall of the cell can be covered with them, 

 they arrange and distribute themselves very equally in this manner, and such cells 

 appear uniformly green. It then seems as if the whole cell-chamber were entirely 

 filled with chlorophyll-granules, but this is not really the case. The central cavity 

 of the protoplasm filled with cell-sap never contains a single chlorophyll body. 



The chlorophyll-granules imbedded in the parietal protoplasm can also undergo 

 the most remarkable displacements, which we will forthwith describe. 



With regard to shape, cells with active protoplasm, containing chlorophyll- 

 granules, exhibit the widest variety. Especially are all imaginable cell shapes 

 to be found in the group of Desmids which live in water : rod-shaped, cylindrical 

 (fig. 25a, k), crescent-shaped (fig. 25a, i), tabular, stellate, tetrahedral, and many 

 others for which it would be hard to find short and suitable names. The Algoe, 

 which to the naked eye seem composed of green threads, are built up of cells which 

 are, for the most part, tubular and cylindrical (fig. 25a, a, 6, and I, m). In Lichens 

 and Nostocacese the cells which form the tissues are spherical; in Mosses and Liver- 

 worts they are pentagonal and hexagonal. 



As already mentioned in former sections, the green tissue in the foliage of 

 Phanerogams is formed, in the majority of instances, of two kinds of cells — of 

 branched cells forming the spongy parenchyma, and of cylindrical cells which con- 

 stitute the palisade tissue (fig. 62, p. 279). The latter are often short, their length 

 being not much greater than their width, but usually they are five or six times, and 

 occasionally even ten or twelve times, longer than broad. In bulbous plants the 

 palisade-shaped cells are arranged parallel to the upper leaf-surface, but in the 

 majority of seed-bearing plants they are at right angles to the upper surface of the 

 foliage-leaf, as shown in the cross-section of a leaf of Salix reticulata, fig. 71 ^ 

 p. 801. The green cells below the epidermis of pines and various firs exhibit a very 

 peculiar form. In contour they appear angular and tabular, and are fitted closely 

 to one another without intercellular spaces. From the cell-walls parallel to the 

 upper surface of the leaf trabeculae project into the interior, by means of which each 

 cell is divided up into niches usually of equal size. Such cells remind one of 

 stables in which the stalls of the different horses are separated by boarded parti- 

 tions. The projecting trabeculae are always so arranged that the entire cell-chamber 

 appears like a group of palisade cells whose side walls separating one from another 

 have been interrupted. These partitions, which, as stated, are to be found in many 

 firs, but also in grasses and many Ranunculaceae — especially in the Monkshood 

 {Aconitum), Peony {Poeonia), and Marsh Marigold (Caltha) — increase the internal 

 surface of the chamber, and this appears to be advantageous, inasmuch as by 

 this means many more parietal chlorophyll-granules can find a place than wouli> 



