OBSERVABLE PROPERTIES OF EACH SPECIES 13 



rare : it happened sometimes that it was impossible to find one 

 single perforation in all the leaves of a specimen. The plants 

 were, in every other respect, quite healthy. In 1903 they were 

 brought into a new greenhouse which was warmer and moister. 

 After a few months the new leaves were abundantly perforated, 

 and from that time on the plant was again deserving of its 

 name (J. V. BURVENICH). 



§ 16. — PLASTICITY {continued). PRIMULA, CAM- 

 PANULA, CRIMSON RAMBLER.— In the preceding ex- 

 amples external causes brought about differences between 

 specimens of the same species. Inversely, specific differences 

 may disappear under the influence of unusual conditions of life. 



EXAMPLES : In some subspecies of Primula sinensis the 

 corolla is white, in others it is pink. Under ordinary circum- 

 stances, the temperature being 10° to 20° C, the colour is quite 

 characteristic of each form. But if a certain pink subspecies 

 is cultivated at a temperature of 30° C, the pink colour dis- 

 appears : the coroUa becomes white and can no longer be 

 distinguished from that of a white subspecies. 



Similar facts have been observed in Campanula. Some 

 species of this genus include blue and white subspecies. A 

 certain subspecies with a blue corolla, when grown at a high 

 temperatmre, is transformed into a white form (variant) which 

 resembles a white subspecies. 



The crimson rambler rose is cultivated at Ghent ^ in many 

 gardens. The flowers are comparatively small, very numerous 

 on each flowering branch and crimson in colour. When an 

 inflorescence (corjonb) is crooked before the buds have reached 

 their full size, the flowers of this branch, when expanded, are 

 qtiite healthy, but white. The crimson rambler is, as it were, 

 transformed into a white rambler. ^ 



§ 17. — PLASTICITY (continued). CALLITRICHE. — In 

 two examples mentioned in § 16 a difference between two 

 (sub) species disappears under conditions which are imusual or 

 even abnormal. Inversely it is probable that, in numerous 

 cases, two specific forms which are hardly different under 

 ordinary conditions of Ufe become distinctly different under 

 unusual circumstances. 



EXAMPLE : In 1911 (in Flanders) the summer was hot 

 and very dry. In dried-up ditches I observed repeatedly 

 the terrestrial form (variant) of Callitriche. The stems were 



1 Probably introduced from England. 



' This fact was observed by Mr COLLUMBIEN, SENIOR, horticultural 

 teacher at Ghent. I repeated the experiment several times with success. I 

 tried to obtain in the same way a similar transformation with two other 

 sorts of roses with pink flowers, but without any effect. 



