Permeability ©9 
cells, as starch was formed in considerable quantities when the leaves 
were provided with these sugars in the dark. Galactose, mannose, 
sorbose, rhamnose and glycerol also entered the cells as indicated by 
the formation of starch, but no starch was formed in leaves provided 
with arabinose, xylose, erythrite, mannitol or dulcitol. However, this 
is not evidence of non-penetration of these substances, as the leaf 
cells may not be able to utilise them in the production of starch. 
The value of similar evidence for the penetration of formaldehyde 
is slight (Spoehr, 1916; Jorgensen and Stiles, 1917). 
(v) Other substances. A number of substances not included among 
the classes already mentioned will combine with some constituent 
of the cell sap to produce insoluble substances which thus become 
visible in the cell as precipitates. The penetration of the purine 
caffeine into cells was noted in this way by Pfeifer, the caffeine pro¬ 
ducing a precipitate with tannin contained in the cell The per¬ 
meability of antipyrin was observed in the same way by Pfeffef. 
Similarly the penetration of substances producing colour in the 
cell can be observed. Thus Miss Williams (1918 a) has shown 
that living mycelia of Peniciliium glaucum and Oidium lactis can 
absorb gold from colloidal solution, but that the gold is absorbed and 
retained by the cell walls which are stained blue. 
'2. Determination of Permeability by Means 
• , of Microchemical Tests 
The entrance of many substances into the cell can be made evident 
by treating the cell afterwards with some microchemical reagent 
which can be used as a test for the particular substance under examin¬ 
ation. By this method Janse (1887 b) investigated the permeability 
of a number of plant cells to potassium nitrate. Following this 
substance the cells were treated with diphenylamine which gives 
a blue colour with nitrates (Molisch, 1883). Cells of Chcetomorpha 
aurea, Spirogyra rtitida, S. crassa, S. communis , Tradescantia discolor , 
Curcuma rubricaulis and Stratiotes were all found to be permeable 
to the inward passage of potassium nitrate. Wider (1887) performed 
similar tests with seedlings of Phaseolus multiflofus , Vida Faba and 
Helianthus annum, testing in addition for potassium in the plants 
with platinic chloride with positive results. This method is obviously 
capable of wide extension as a qualitative test of permeability and 
absorption. It does not lend itself in its present form to quantitative 
measurements of either. ... 
