ENVIKONMENT AMONG ANIMALS AND PLANTS 327 



develops under normal stimuli, abnormal stimuli experimentally 

 induced may be followed by every kind of result from the most 

 extreme to the most insignificant changes both of form and of 

 life-cycle. We may note the results of a few of the vast number 

 of experiments which have been made. With regard to plants 

 it has been shown that ' each developmental stage depends upon 

 special external conditions, and in cases where our knowledge 

 is sujBQcient, a particular stage may be obtained at will. In the 

 Green Algae, as in the case of the Fungi, we may classify the 

 stages of development into purely vegetative growth (growth, 

 cell-division, branching), asexual reproduction (formation of 

 zoospores, conidia), and sexual process (formation of male and 

 female sexual organs). By modifying the external conditions, 

 it is possible to induce algae or fungi ... to grow continuously 

 for several years, or, in the course of a few days, to die after an 

 enormous production of asexual or sexual cells. In some instances 

 even an almost complete stoppage of growth may be caused, 

 reproductive cells scarcely being formed before the organism 

 is again compelled to resort to reproduction. Then again the 

 sequence of different stages of development may be modified 

 as we desire.' ^ 



Another kind of experiment shows that foliage shoots can be 

 converted into runners and vice versa ; it is, for instance, possible 

 to induce a germinating tuber of the potato to form foliage shoots 

 under the influence of a higher temperature. The transfer of 

 plants from one environment to another is often followed by 

 remarkable changes. MacLeod, for instance, states that a species 

 of Philodendron which has large leaves pierced with round holes 

 was cultivated for many years in the Botanical Gardens at Ghent 

 in a greenhouse which was rather cool and dry. The holes were 

 found to be rare ; at times it was not possible to observe a single 

 perforation in any of the leaves of a specimen. In every other 

 respect the plants were healthy. The plants were later transferred 

 to a greenhouse that was warm and moist and after a few months 

 the new leaves were found to be abundantly perforated.^ Bonnier 

 made some very interesting observations on the dandelion. He 

 found that the plant, when sown at a high altitude in the Pyrenees, 

 produced very short stems with hairy, dark green leaves and 



* Klebs, ' Influence of the Environment ', in Darwinism and Modern Science, 

 p. 227. 2 MacLeod, Quantitative Method, p. 12. 



