94 MORPHOLOGY OF THE CELL. 



varies with the age of the cell and with the time of year, as 

 shown in the figures. 



283. Anilin blue is the best pigment for bringing out the 

 form of the callus clearly. If, as Russow l recommends, its use 

 be supplemented by that of Schulze's iodide, the callus may be 

 seen to be made up of at least two portions, distinguished by 

 the depth of color. In young and active cribrose-cells the callus 

 usually appears to be a gelatinous layer on each side of the sieve- 

 plate ; in most old cells it is no longer seen. 



284. Contents of the cells. In the younger and active state 

 just referred to, the cells contain a wateiy liquid which holds 

 more or less granular matter, and the walls are lined by a delicate 

 film of protoplasmic substance. That the callus is also of a pro- 

 toplasmic nature is not clear, although some of its reactions 

 suggest this. It frequently contains minute granules of starch, 

 which sometimes give a bluish-brown color with iodine, like 

 starch which has been acted on by diastase. Russow thinks that 

 a ferment is present in the cells in their active state. When 

 old, most cells lose not only the callus but also the greater part 

 of their other contents. In active cells there are frequentlj* 

 found very small but brilliant globules which are albuminoidal. 

 All the contents above mentioned vary within certain limits at 

 different periods of the year. 



285. The sieve-cells of the higher cryptogams have been 

 shown by Janczewski 2 to be nearly if not quite imperforate at 

 all seasons. In gymnosperms, they pass through two periods : 

 the first, or the evolutive, in which the plates produce the callus, 

 the cells themselves containing parietal protoplasm ; the second, 

 or passive, stage, in which the protoplasm disappears entirely, 

 and communication between the contiguous cells occurs. In 

 monocotyledons and dicotyledons the cells have four periods; 

 namely, the evolutive, the active, the transitory, and the passive. 



IV. Latex-cells, Latex-tubes. 



286. Certain plants when wounded exude a milky juice known 

 as latex. They belong to widely separated orders ; for instance, 

 to Papaveraceoe, Campanulaceae, Asclepiadacese, Urticace, etc. 



The cells in which latex occurs are characterized by a soft- 

 ness of cell-wall which renders them easily compressible ; hence, 



1 Annales des Sc. nat, bot., se>. 6, tome xiv., p. 167. 

 8 Annales des Sc. nat. bot,, ser. 6, tome xiv., p. 50. 



