The Origin and Occurrence of Rubber. 179 



APPEARANCE OF RUBBER IN RICHLY LOADED TISSUES. 



A section taken through any young stem of a field plant after some 

 period of drought will give a typical appearance (plate 42, fig. 7). All the 

 cells of the conjunctiva appear to be filled with a gray substance. A good 

 deal of it will have been swept out of ruptured cells by the knife-edge and 

 agglomerated, the resulting masses having irregularly rounded outlines, 

 with strands stretching here and there, still attached to the tissue. These 

 masses, seen obscuring the pith to some extent in plate 43, fig. 2, and the 

 dense cell-contents stain deeply with alkanet, the stain being more bril- 

 liant if the sections have been previously boiled in a 10 per cent solution 

 of caustic potash. If the sections have not been acted on by alcohol or 

 potash drops of yellow resin will be seen in the resin-canals, but nowhere 

 else, except accidentally. 



Closer examination of the rubber within the cells shows that the mass 

 is not homogeneous, and does not entirely fill the cavity. It may form a 

 heavy layer about the wall, leaving a more or less irregular space within, 

 or, if apparently filling the entire cell, it will contain numerous spherical 

 spaces (plate 41 , figs. 1,2). Sections which have lain in glycerin may show 

 the masses tobe contracted, owing to a plasmolytic action upon them, from 

 which it is to be inferred that they have a considerable water-content, 

 held within the vacuoles, in part at least (plate 41 , fig. 3) . The rubber may 

 also accumulate as a round drop within the vacuole of the cell (plate 42, 

 fig. 7), its size depending upon the age of the cell. Plasmolysis shows 

 further that all the parenchyma cells are not equally densely filled, though 

 of the same age. This is often conspicuously the case when the cells of 

 the cortex and those of the adjacent parenchyma rays are compared. In 

 the cortical cells the rubber forms a dense rounded drop (plate 42, fig. 7), 

 while the cytoplasm may be seen between it and the cell wall. In paren- 

 chyma-ray cells the rubber mass is frequently irregular, full of irregular 

 vacuoles, and the cytoplasm appears usually to have shrunk away with 

 it. In the parenchyma-ray cells in some preparations it is quite as regular 

 as in the adjacent cortical cells, but appears to be more dense, owing to a 

 very much larger number of minute spaces. This difference, in one form or 

 the other, is quite constant, and seems to indicate that the rubber-content 

 of the cortical cells is higher than that of the adjacent parenchyma-ray 

 cells. 



Cortex which has been cut out en masse by inner periderm also con- 

 tains rubber. In the cells of this tissue it has a still different appearance, 

 being segregated into droplets of various sizes, in a fashion to suggest the 

 analogous appearance of dead protoplasm. In newly formed cork-cells 

 proper, just outside of the periderm, a different behavior is seen. 



BEHAVIOR OF PERIDERMAL DIVISIONS TOWARD RUBBER. 



Since the secondary cortical cells in field plants contain a large 

 amount of rubber in the condition described, the fact that the cork-cells 

 immediately outside of the actively dividing suberogenous cells may contain 

 no rubber at all, or only occasionally a small amount, calls for explanation. 

 The suberized walls of the cork take up alkanet readily, so that, after 



