34 PHARMACEUTICAL BOTANY 
the stigma and style to the ovule below. In the former instance, 
negative chemotropism is illustrated, while in the latter, positive 
chemotropism is shown. 
SITOTROPISM is the reaction of living matter to the influence of 
food. When the response is by motile organisms or motile 
cells, it is called setotaxy. Hertwig found that if a fine capillary 
tube be filled with a 1 per cent solution of asparagin or beef 
extract and held in contact with a drop of water containing 
certain bacteria, a mass of these soon plugged the mouth of the 
tube. His experiment shows that these organisms moved from a 
poorer to a richer nutrient medium in response to a positive 
sitotaxic influence. 
OxyTRopisM is the response to the stimulating influence of 
oxygen. We see evidence of this everywhere in nature. No 
living thing can continue to exist without this element. A 
mistaken idea is often prevalent regarding obligate anaérobic 
bacteria. Like all other bacteria or organisms, these plants 
require oxygen but can only assimilate it in its combined form. 
The tetanus bacillus is a good example. Aérobic bacteria, on the 
other hand, require free (uncombined) oxygen for assimilative 
purposes. Thus the tetanus organism (an anaérobe) grows in 
the depth of culture media, whereas the tubercle bacterium 
(an aérobe) grows only on the surface. 
HyprotropisM is the response of protoplasm to the stimulus 
of water. This reaction is seen in both positive and negative 
phases in the slime molds. ‘The vegetative stage of these lowly 
plants is characterized by a naked, many nucleated mass of 
protoplasm, confining itself to the moist crevices of rotten logs, 
etc. until the surface of the substratum becomes wet, when and 
only when it will emerge (positive reaction). As soon, however, 
as its fruiting stage begins, the whole protoplasmic mass wells up 
from the substratum, away from moisture (negative reaction). 
The roots of young seedling plants show positive hydrotropism by 
growing toward moisture in the soil. 
RHEOTROPISM is the response made by roots and _ slime 
moulds to currents of water. 
HeioTropiso is the response of living substance to the stim- 
ulus of sunlight. 
