116 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
ability for the new conditions, but this advantage is offset by the fact 
that other members, such as the root, have also undergone a_restriction 
with the general result of dwarfing and no increase in efficiency in dealing 
with the newly encountered conditions. 
Over 100 species of seed-plants were grown in complete darkness and 
under faint illuminations in the experiments upon etiolation completed 
three years ago. The action of light was seen to act as a stimulus set- 
ting in action morphogenic processes of unusual character, by which the 
stems of some species were enormously thickened, in others unduly 
elongated, while nearly all showed various useless atrophies and hyper- 
trophies in which the supply of reserve food-material was used to no 
perceptible advantage. Illustrations of a similar character might be 
multiplied indefinitely. 
Not only are the direct ontogenetic and morphogenetic responses of 
plants to environmental forces not necessarily of an adaptive character, 
but it is impossible to connect some of the most highly specialized and 
heritable structures with the supposedly causal conditions which they 
meet. Spines and glochids do lessen the ravages of grazing animals 
upon cacti to some extent, but such structures seem to be induced by 
aridity or poverty of available water, and the deterioration of leaves 
and branches has been carried farther in a dozen native species, with 
the result that we have that many nearly unarmed forms in American 
deserts. This water-relation alone may not be taken to account for the 
form and habits of the cacti, since many members of the family inhabit 
tropical forests and regions of pronounced rainfall. 
’ In the case of the cacti, the forms and capacities which have been 
taken on differ so completely from those of any possible ancestor to 
which they might be traced, that it is extremely difficult to place them 
in a phylogenetic system. The appearance of various members of this 
family in widely separated regions and under such greatly different cli- 
matic conditions suggests a derivation from some group of morphological 
types having within them the possibility of vegetative development with 
reduction of the branching of the shoot and the increase of the body 
resulting in succulency and capacity for water-storage. The appearance 
of such notable differentiation within such a comparatively brief time 
is perhaps to be taken as one of the most rapid and notable evolutionary 
developments which has yet been brought up for consideration. 
It has been shown above that the changes undergone by the shoot of 
a plant when it is brought into an unusual environment may or may not 
lead to increased efficiency in meeting the conditions which induce them, 
the entire matter resting upon the fact that when a stimulus consisting 
of a change in intensity of temperature, light, soil-solutions, or humidity, 
is brought to bear upon a plant it responds as a living machine, while 
at the same time the direct effect of the external force upon the parts of 
