1913] KNIGHT & CROCKER—TOXICITY OF SMOKE 345 
summarized the effects of poisons upon the development of plant 
structures. Under modifications due to increased cell pressure, he 
mentions inhibition of growth in length, favoring of growth in 
diameter, splitting of the tissues with formation of rifts, lenticular and 
intumescence formation, and maceration in the living body. Other 
modifications that he relates to the rise of turgor are collenchyma 
formation, thickening of the epidermis, vacuolization of the nucleus, 
and fusion of the nuclei. 
These two lines of general evidence are very interesting in con- 
nection with the response of the pea seedling, but after all they leave 
essentially untouched the real solution of the physiology of the 
response. This becomes more evident when it is remembered that 
our work shows three distinct types of response of this seedling to 
atmospheric impurities: (1) that produced by ethylene, acetylene, 
propylene, illuminating gas, various sorts of smoke, and possibly 
methane, and characterized by decreased rate of elongation, swel- 
ling, and diageotropism; (2) that produced by ether, chloroform, 
benzol, toluol, thiophene, xylol, cumene, and other substances, and 
characterized by decreased rate of elongation and swelling but not 
diageotropism; (3) that produced by ethyl alcohol, propyl alcohol, 
pyridine, hydrogen sulphide, hydrogen chloride, and other sub- 
stances, and characterized by decreased rate of elongation, but 
neither swelling nor diageotropism. A careful detailed study of 
osmotic pressure, permeability, and metabolic (enzymatic, acid, 
respiratory, etc.) behavior of the seedlings, in each group of poisons 
might throw much light on the internal intimate physiology of the 
three types of response. A detailed study in this line might also 
relate diageotropism more closely with physical and chemical - 
characters of the organ. This organ furnishes especially desirable 
material for such a study because of the ease with which it is 
changed from a negative to a diageotropic organ. 
2. EFFECT OF TOBACCO SMOKE 
Mottscu has shown that tobacco smoke is extremely toxic to 
many plants. In his first paper (22) he reports the effect of 
tobacco smoke on various seedlings and microorganisms, and in a 
second paper (23) the effect upon adult plants. A third paper (24) 
