THE MOLECULAR FORCES IN PLANTS. 133 



protoplasm is impermeable to the pigment. If we carefully rinse 

 pieces of fresh beetroot in order to remove the sap coming from 

 the cut cells, and then lay them in water, they yield no colouring 

 matter, even after remaining in the fluid for an hour or more. 

 If, however, the cells are first killed by heat, and then laid in 

 w;iu>r at the ordinary temperature, their colouring matter rapidly 

 escapes. 



Pieces of turnip are killed by immersion in water at 60 C., and s 

 then laid in beetroot juice, together with pieces consisting of 

 living cells. The former will be found within twenty-four hours 

 to be coloured red through and through, while the pigment has 

 not penetrated the latter. 



52. The Effect of Mechanical Injury. 



Although it is known to everybody that plant structures can 

 survive slight pressure or slight tension without injury, but suffer 

 destruction of their molecular constitution if submitted to consider- 

 able mechanical injury, a few not quite unimportant experiments 

 may nevertheless be made which clearly bring out this last fact. 



Water at 15-20 C. is poured over some potato starch; An- 1 

 equal quantity of starch is mixed with clean quartz: sand, and 

 ground down as thoroughly as possible in a mortar. Water is 

 then poured over this also. After a few hours we filter off the 

 fluids. Testing with Iodine, we find that the water has ex- 

 tracted granulose from the starch ground down with sand, but we 

 can detect no granulose in the other case. The mechanical injury 

 has destroyed the molecular organisation of the starch grains, 

 which consequently give up granulose to the fluid, while the un- 

 injured grains cannot do so. 



If we violently squeeze between the fingers the blade of a leaf 

 of Begonia manicata, the injured parts at once become brownish 

 in colour. Microscopic examination of tangential sections of the 

 crushed portion teaches that the chlorophyll grains, which in the 

 normal cells are beautifully green, have become discoloured. The 

 pressure has destroyed the protoplasmic constituents of the cells. 

 They have become permeable to the acid cell-sap, and this has 

 caused the decomposition of the chlorophyll. 1 



1 See Detmer, Botan. Zeitung, 1886, No. 30. 



