GROWTH AND MOVEMENT 439 
stimulus, of it may make possible the stimulating action of other sub- 
stances within the plant, which, but for their increased concentration, 
would not act so. Unquestionably other causes than lack of water 
around the roots of a plant may call forth such structures, as is well seen 
in the case of bog plants. Indeed it has become customary to speak of 
" physiological " drought as the cause of xerophytic structure, when 
physical drought is obviously out of the question. This may be taken 
as a convenient expression for some difficulty which prevents the plant 
from admitting a sufficient amount of water, such as the poor develop- 
ment of the root system. Whatever does this will tend to dwarf or other- 
wise transform the aerial parts, either as the plain lack of water does, or 
possibly in quite different and unrecognized ways. (See further, Part 
III on dwarfing in bogs.) 
Recent investigations are bringing to light some new causes for the imperfect 
development of plants, which probably is due primarily to an effect on the roots. 
It is found that the sterility of some soils is due to the presence in these soils of organic 
substances, which are partly soluble, so that a watery extract of such soils, when 
used as a water-culture medium, acts as badly as the soil itself. Furthermore, 
these substances can be removed in large part by adding some finely divided ma- 
terial like lampblack to the liquid and then filtering it out. The filtrate may then be 
used without detriment to the cultures. Still further study makes it probable that 
these substances originate in large part from the plants which have previously grown 
in the soil. The necessity for the rotation of crops on any field has long been known. 
The reason has been assumed to lie in the exhaustion of the materials which are 
supposed to be necessary for the nutrition of the plants. Without denying that there 
may be something in this assumption (it is nothing else at present, because the ex- 
perimental evidence upon which it rests is faulty), it seems now much more likely 
that the chief, cause is to be found in the excretions from the roots of the previous 
crops and the products of their decay in the soil. It has been shown that though 
the mineral salts of a culture solution be maintained unchanged, the water becomes 
more and more unfit for use with repeated cultures of the same species, and that 
this impairment may be remedied by treatment with lampblack as above described, 
though the content of salts be not altered. Water cultures, to which have been 
added various organic substances that might be produced, or are known to occur 
in plants, have shown like injuries to the plants, and though the amount of the 
deleterious substances occurring in nature is too small for direct analysis, their 
general character may be ascertained by further experimentation in this way. 
Mechanical agents. Pressure and tension have evident influence 
on the development of mechanical tissues. The encasing of a stem in 
a plaster cast, so that as it thickens it will compress itself, leads to 
changes in the structure within the zone of compression and especially 
just beyond the margin. Continuous tension seems to bring little if any 
