64 
PHARMACEUTICAL BOTANY 
agencies from anthers to stigmas of certain plants of different species 
will not germinate but when they are carried from one plant to an¬ 
other of the same species or variety they readily send their pollen 
tubes through 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. 
Hertwig found that if a fine capillary tube be filled with a i 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 sitotropic 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 ele¬ 
ment. A mistaken idea is often prevalent regardingobligate ana¬ 
erobic 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. Aerobic bacteria, on the 
other hand, 'require free (uncombined) oxygen for assimilative pur¬ 
poses. Thus the tetanus organism grows in the depth of culture 
media, whereas the tubercle bacterium (an aerobe) grows only 
on the surface. 
Hydrotropism 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 ef protoplasm, con¬ 
fining itself to the moist crevices of rotten logs etc. until the surface 
of the substratum becomes wet when and only when it will emerge. 
As soon, however, as its fruiting stage begins, the whole protoplas¬ 
mic mass wells up from the substratum, away from moisture. The 
roots of young seedling plants show positive hydrotropism by growing 
toward moisture in the soil. 
Heliotropism is the response of living substance to the stimulus of 
light. The stems of higher plants tend to grow toward the light 
and are, therefore, positively heliotropic, whereas the roots grow 
away from the source of light and so are negatively heliotropic. 
