156 The Mode of Examininrj the [^"^Zl Ml%hT?l?u! 



occurring in the aleuron grains of fatty seeds, as Kicinus, wliicli are 

 brought into \aew by dissolving the fatty matter by means of ether. 

 Coloured crystalloids are described by Naegeli as occurring in the 

 petals of Viola tricolor, Orchis, &c. The aleuron grains, so called 

 by Hartig, are rounded granular formations found in ripe seeds : 

 as the grains are altered by the action of water it is difficult to 

 examine them. 



Starch is another substance always found in the protoplasm; 

 the granules increase in size as long as the protoplasm remains in 

 the cell, but when it disappears the starch grains mix with the cell- 

 sap, and all growth stops. The grains present well-marked differences 

 in appearance, some of the grains being very characteristic. 



The Cell-sap. — By cell-sap we mean the fluid contained in the 

 vacuoles of the protoplasm. It seems to be essential to the growth 

 of the cell, and contains many substances in solution. We may 

 have organic acids, mineral salts, alkaloids, &-c., dissolved in the 

 cell-sap, and invisible; but frequently we have colouring matters, 

 oil, or crystals in the cell-sap. Piaphides seem also to be products 

 of the cell-sap, as well as other substances, such as sugar. The 

 last stage in the life of the cell is the disappearance of all the 

 contents, and nothing remains but the dry cell-wall, more or less 

 modified to adapt it to the position it occupies in the j)lant and the 

 function it has to perform. 



Let us now pass to the consideration of the tissues. We may 

 consider a tissue as any aggregation of cells governed by the same 

 laws of growth. In general we have the tissue formed by the 

 repeated division of the mother-cell at the growing part of the 

 plant ; but in a few rare instances we may have cells originally 

 free becoming fused together to form a tissue. This is seen in 

 certain of the Algae, as Pediastrum, Hydrodictyon, &c. When the 

 walls of the cells forming a tissue are examined, we see but a single 

 plate or lamella. This is well seen in the young growing cells at 

 the point of the root, as in the White Mustard. When the cells 

 become older and thickening occurs, then it is found to be deposited 

 on each side of this lamella, in the inside of the wall of the cells. 

 When thickening has thus taken place, the appearance becomes 

 very deceptive, and we might think that the thickening layers were 

 the true cell-walls, while the central lamella was a layer of inter- 

 cellular substance. By a careful use of reagents we can separate 

 these two elements, the lamella being soluble in sulphuric acid, 

 while the thickening deposits can with care be dissolved in chlorate 

 of potash and nitric acid. When the thickening deposits have been 

 dissolved we can isolate the delicate lamella or cell-wall. The 

 so-called intercellular substance, so well seen in many of the Alg<)3, 

 is a gelatinous degeneration of tlie lamella. The lamella thus be- 

 comes greatly thickened, and the masses of thickening matter inside 



