SEPTEMBER 8, 1899. ] 
probably dependent upon their own consti- 
tution. Fischer’s researches seem to show 
that the molecular relation between the 
two are as intimate as those between a key 
and the lock whose wards it must fit before 
the position of the parts can be altered. If 
this proves to be true, we shall look for a 
better understanding of the processes of 
digestion with the further extension of the 
stereochemistry of the nitrogen compounds. 
Again a knowledge of the physiological 
action of definite radicles, which may 
differ according to their position in the 
molecule, is being reached by determining 
the effect of the introduction of a certain 
radicle or of a change in its position. The 
ability to alter chemical structure at will 
by known reactions puts it into our power 
to ascertain how each change affects the 
protoplasm. Thus in the phenols, a series 
of compounds allied to the tertiary alcohols, 
of which the so-called ‘carbolic acid’ is a 
familiar example, True and Hunkel find that 
the introduction of the nitro group (NO,) 
or the methyl] group (CH,) into the benzene 
nucleus increases the poisonous effect, 
while an increase in the number of hydroxyl 
groups (OH) or nitro groups (NO,) has 
little or no effect. 
PHYSIOLOGICAL MORPHOLOGY. 
Within the past decade attention has 
been especially directed to the causes which 
affect the development of plants and 
determine both form and structure. A 
moment’s thought suffices to impress upon 
any one the fact that a great number and 
extreme variety of external agents are act- 
ing and interacting in most complex fashion 
upon all plants. Some of the more obvious 
of these groups of external causes are even 
popularly recognized. Thus one hears it 
said that poor soil and scanty water is the 
cause of the dwarfing of plants, which 
under better conditions attain a greater 
stature. 
SCIENCE. 323 
Such apparently obvious deductions may 
be correct, or may not be, but satisfactory 
and accurate analysis of the effects of ex- 
ternal agents is a problem of the utmost 
difficulty, because it is well-nigh impossible 
to alter experimentally one condition with- 
out really altering others at the same time. 
The solution of the problems of physio- 
logical morphology is, therefore, only to be 
attained by the most assiduous care in ex- 
periment and induction. 
Morphology.—In illustration of these prob- 
lems I may refer to the recent studies made 
by Klebs on the external factors which con- 
trol various reproductive processes among 
the alge. By experimental analysis he 
has sought to determine the bearing of 
light, temperature, density of medium, and 
various other agents upon the production 
of zoospores and gametes. These studies 
have shown that it is possible to call forth 
a definite and very complicated physio- 
logical process, of far-reaching conse- 
quences, by appropriate changes in the en- 
vironment. How they operate remains yet 
to be explained. 
In the higher plants the investigations 
of Goebel and many others have shown the 
possibility of controlling growth and devel- 
opment in asimilar way and to aremarkable 
extent. The relation between the different 
members of a plant has also been exploited, 
largely within the past decade, although 
its beginnings were long ago. The study 
of correlations has cast much light upon 
the causes of form, and has made more im- 
pressive than ever the wonderful plasticity 
of plants. 
Correlations. — Qualitative correlations, 
particularly, offer an inviting though diffi- 
cult field for investigation. I need only 
mention a few examples of such correla- 
tions. Upon the removal of the terminal 
shoot of a pine, one or more of the lateral 
shoots erect themselves and undergo appro- 
priate changes in mode of internal growth 
