152 THE DESTRUCTIVE INFLUENCE OF 
increases as their life becomes more sluggish. Thus 
Spallanzani says, “ One may say that in winter plants 
live less fully than at other seasons, and during this 
period they are also much less liable to perish when 
they are plucked from the ground or unduly pruned, 
than if they had been treated in the same manner 
during summer.” 
Again, whilst a difference of the same kind may in 
part be cited as the cause of the less injurious effect 
of heat upon seeds and plants as compared with that 
which it exercises over eggs and animals respectively, 
Spallanzani believed that the greater tenacitv of life 
shown by seeds is only in part due to the fact that the 
outer coats of most seeds are much harder than those 
of eggs. Thus, the envelopes of some seeds which are 
only killed at a temperature near 212° are not harder 
than the shell of an egg, which is nevertheless killed 
at the much lower temperature of 140° F. This 
difference is explicable, according to Spallanzani, 
rather by the fact that the fluids contained within the 
egg are so much more abundant than those within 
the seed. The greater moisture of the animal embryo 
causes it more rapidly to experience the full effect of 
the heat, so that with a short exposure to a given 
temperature the egg may be easily killed whilst the 
seed escapes.* 
* Spallanzani’s argument thus naturally suggests the notion that many 
