RESISTANCE TO COLD. 489 



of one species are content with a low temperature, while those of anotlier re(iuire 

 much greater heat, although the eye can distinguish no difference in the structure 

 of their coat, in their manner of storing reserve food, or in the structure of their 

 embryos. The same may be said of the freezing of plants. Many Californian and 

 Mexican Pines {Pinus) are very like those of Northern and Central Europe, and 

 yet the one will be frozen to death as soon as the temperature sinks below freezing 

 point, while the other can sustain winter temperatui-es of -20° C. without injury. 

 There seems to be no reason why the South European Junipers, Juniperus Oxycedrm 

 and phoenicea, which are apparently of the same structure as the similar species 

 Juniperus nana and Sabina, should not flourish equally well on our mountain 

 heights in the Central Alps, where the latter cover whole mountain peaks and send 

 their roots into ground which is covered with snow eight months every year, and 

 is frozen hard for months together. The common Ivy (Hedera Helix) grows in 

 Central Europe without any protection from the fairly severe cold of winter; the S. 

 European Ivy, Hedera poetarum, which is very similar to the common species, but 

 can be distinguished from it by several external characteristics, requires a protecting 

 roof in the gardens of Central Europe if it is to survive the winter unkilled by the 

 frost. The same is true of two closely allied species of Marigold, viz. Caleiuhda 

 arvensis and fidgida, the former growing in Central, the latter in Southern 

 Europe. In 1874 I sowed seeds of Calendula arvensis from the Rhine district 

 side by side in the same garden-bed with seeds of Calendula fulgida, which had 

 been gathered in Sicily. Very luxuriant plants which flowered in profusion grew 

 up from both kinds of seeds. The first frost in that year in the place where the 

 experiment was made occurred on October 25th. Calendula arvensis was not 

 injured; its foliage was fresh and green, and remained in this condition during the 

 following days, although, until November 2nd, the temperature fell every night 

 from —1-5° to —2-5° C, and in the morning the stem, leaves, and flowers were 

 studded with hoar-frost. Calendula fulgida, on the other hand, was destroyed by 

 the frost on the night of the 24th-25th October. Its leaves and stems withered and 

 turned brown, and exhibited all the symptoms observable in death b}^ freezing. In 

 1864 I found a Cytisus on the rocky shores of the Adriatic Sea at Rovigno, which 

 closely resembled the wide-spread Cytisus nigricans of Central Europe, but whicli 

 had certain distinguishing features. I named it Cytisus australis. Some of its 

 seeds were collected, and from them strong young seedlings were obtained in the 

 following year. These were planted in the Botanic Garden at Innsbruck with 

 some seedlings of Cytisus nigricans of the same age from the Danube valley, neai- 

 Mautern, in Lower Austria. Both grew under identical external conditions, and 

 appeared to be equally vigorous. But during the winter the plants of the Cytisus 

 from the shores of the Adriatic were killed by the frost, while those of the Cytisus 

 from the Danube valley remained healthy and strong. The experiment was repeated 

 with the two plants in the following year. Young plants were again raised from 

 seed, but this time those from the Adriatic coast were protected against the cold, 

 and in this manner they survived the winter without harm. Two years later botli 



