AetOLS] KNIGHT & CROCKER—TOXICITY OF SMOKE 367 
If it is carbon monoxide, why not generate it chemically and thereby 
avoid ethylene? Again, it is a rather common practice to have 
the heating furnace in more or less open connection with the 
greenhouses. If one recognizes the probability of the dry distilla- 
tion gases escaping from a furnace, along with the extreme toxicity 
of ethylene, he can see the need of abolishing this practice. 
So far as known, the etiolated epicotyl of the sweet pea is the 
most sensitive plant organ to ethylene. As has been stated, it is 
inhibited in growth by 1 part in 10,000,000 of atmosphere. The 
open flower of the carnation is only a little less sensitive. In our 
original measurements (6), which were made on plants that had been 
bearing flowers for several months, 1 part in 2,000,000 of atmosphere 
“put the open flower to sleep” in 12 hours. Some later measure- 
ments with plants soon after they had begun to flower showed that 
I part in 3,000,000 of atmosphere caused the same response. While 
the open flowers on these younger plants were much more sensitive 
to ethylene, the buds proved much more resistant than the buds 
of plants longer in bearing. 
If, in the few cases tested, two such sensitive plant organs have 
been found, it is probable that many more exist. So far as we know, 
there is in nature no special absorbent for ethylene, also no cycle for 
the gas, as there is for carbon dioxide and oxygen. Even if both 
existed, one doubts if 1 part in 10,000,000 would lead to a with- 
drawal. Processes of civilization are continually adding to the 
ethylene in the atmosphere, as burning of all carbohydrates, burn- 
ing of coal (?), escaping of artificial illuminating gas, producing of 
gas in the beehive method of coking, escaping of certain sorts of 
natural gas, and probably other processes. Having no estimate 
of the total additions from these sources, one cannot calculate 
whether accumulation in the atmosphere up to a danger point is 
likely to occur. 
The etiolated epicotyl of the sweet pea is a very delicate test 
for the heavy hydrocarbons, especially ethylene. One of the papers 
to be published later will show that under proper application it is 
also a very reliable test for this group of substances. It could be 
used to determine the presence or absence of this group of gases 
in coal smoke, gas from coal (28), and natural gas, where gas 
