How Plants fjroic. 



7y 



end to end and fixTnly fixed together. The wall of the cell is 

 coated inside by a thickish fluid in which fine granules float, 

 which fluid lies like a sheet of jelly on the wall. It may be 

 made to contract and solidify by applying spirits of wine, drawing 

 itself up into a more or less regular mass in the centre of the 

 cell, and at the same time bringing away the green band from the 

 wall. Chemical tests show that this internal thickish fluid is of 

 different composition from the membrane forming the wall of 

 the cell, being of a nature more allied to animal substance ; and 

 it appears to be enclosed by a soft and gelatinous layer, which, 

 during the life of the cell, forms a lining to the outer cell- 

 membrane, but if contracted, as when spirit of wine is applied, 

 shrinks up, and necessarily crowds together all the rest of the 

 contents of the cell within it. The thick fluid is called the 

 ^^ pivtoplasm," meaning the orif/inal substance out of icliich the 

 orqans are made, since all the solid parts are formed out of this. 

 Ttie gelatinous layer which bounds it is called the iirimordial 

 utricle of the cell, because it is the first structure that exists, 

 but the term formative layer will be simpler and better for us. 



Having made acquaintance with these structures inside the 

 cell, which exist in all young cells, and remain as long as they 

 retain the power of increase, we can now understand how cells 

 divide. The filaments of most Confervoids grow by simply in- 

 creasing in length, which is effected by the cells dividing cross- 

 ways in the middle, each half growing to the length of the 

 parent. In the Spirogrjrai and other kinds the mode of the divi- 

 sions has been accurately traced. 

 The lining of the cell-mem- 

 brane, the formative layer with 

 the protoplasm inside, becomes 

 gradually parted into two por- 

 tions, just as if a string had 

 been twined round it and was 

 gradually pulled tight, as when 

 a rocket -case is "choked." 

 This produces a fold in the 

 place where the division takes 

 place ; \.\\e formative layer when 

 half divided may be compared 

 to an hour-glass, and when 

 quite divided it forms two bags 

 instead of one, the new ends 

 of the two bags being in contact. 

 Wliilc tliis is taking place, the 

 whole outer surface of the for- 

 mative layer produces a sheet 



Fiff. 



A, 



Cell of Spirogyra dividing into two hy tlio 

 formation or a parlillon. 11, tbe stum^, ireuted 

 with tincture of iodine to consulate tlif " [iro- 

 toplasm." Magnified 250 diameters. 



