THE AMERICAN BOTANIST. 51 



ing of the plants' structure. The fact is this thistle is dioe- 

 cious, that is the staminate and pistillate flowers are borne in 

 separate heads. Only the pistillate flowers produce seeds, so 

 that anyone happening to examine the staminate flower-head 

 world hastily conclude that the plant was not producing seeds. 

 There is a noticeable difference in the shape of the two kinds 

 of flower-heads, the staminate being rather longer than the 

 other. Rarely the heads are incompletely dioecious and have 

 the two kinds of flowers mixed in the same head. 



Origin of Thorns. — It seems to be tacitly assumed by 

 the majority of writers that the changes undergone by a meso- 

 phytic species when grown in an arid situation are of such 

 nature as to fit the the individual better for living under the 

 drier conditions. This does not always follow. The form 

 assumed by a shoot in a dry atmosphere is the result of growth 

 conditions established by the balance among the absorbing 

 organs, the conducting stems and the transpiratory surfaces. 

 * * * Not only are the responses of plants to environ- 

 mental forces not necessarily of an adaptive character, but it 

 is impossible to derive some of the most highly specialized 

 and heritable structures from the supposedly causal conditions 

 which they meet. Spines and spiculate do lessen the ravages 

 of grazing animals upon cacti to som.e extent, but these 

 structures are undoubtedly a direct and accumulated response 

 to aridity or poverty of available water, and in a dozen spe- 

 cies the deterioration of leaves and branches has been carried 

 so much further that we have many spineless or nearly un- 

 armed forms in American deserts which suffer variously or 

 not at all from animals. * * * 'pj-jg entire matter rests 

 upon the fact that when a stimulus, consisting of a change 

 in intensity, light, soil solutions or humidity is brought to 

 bear on a plant it responds as a living machine. — Dr. D. T. 

 MacDougal in Plant World. 



