934 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
hard to see why this type should obtain so largely in the 
S.W. district. There is, to begin with, no winter worthy of the 
name ; and therefore annuals would, when becoming acclimatised, 
probably cease dying at the end of the year, because there is 
no reason why they should. Certain European annuals of cultiva- 
tion have, in fact, become perennials.” 
Now the annual, biennial, or perennial character of plants is 
often regarded as specific; but it is one which can be easily 
changed, and may then become hereditary: thus the garden form 
of the carrot is now biennial, but normally, as a wild plant, 
Daucus Carota, it is an annual. This is simply the result of 
sowing the seed of the original wild form late in the season. 
The consequence was that the plants did not blossom till the 
following year. Then, by selection, this biennial feature has 
been fixed, and is now hereditary. 
Poa annua, if grown in plenty of moisture, at once becomes a 
perennial; as it does also on the Alps, just as several other 
annuals at lower altitudes, as well as latitudes, become perennial 
when growing at higher altitudes and latitudes. 
Mr. Th. Holm * has recorded a number of examples of American 
plants which are ordinarily annuals, but become perennial under 
exceptional conditions. They include Hypericum nudicaule, 
Delphinium Consolida, Cyperus flavescens, Carex cyperoides, and 
species of grasses and Crucifers which are annuals in Europe, 
but perennials in the United States, and particularly so near 
Washington. For example, Arabis dentata, which is typically 
biennial, and A. /yrata, which is normally annual or biennial, 
have formed perennial specimens. On the contrary, A. levigata, 
said to be perennial by Hildebrandt, is not at all rare as a biennial 
near Washington. 
This change in duration may be accompanied by a change in 
the period of flowering ; or the period of flowering may change 
without a plant altering its duration of life. These alterations 
may become permanent. Thus, Sir J. D. Hooker? noticed how 
the stock and mignonette become perennials in Tasmania; on 
the other hand, the castor-oil becomes an annual in England. 
As a remarkable instance of a plant having undergone a 
complete change of season in flowering, Ovalis cernua may be 
* “On the Vitality of some Annual Plants,” Amer. Journ. Sci. xlii. 1891, 
p. 804; also Rev. gén. de Bot. 1892, p. 364. 
t Animals and Plants under Domest. ii. p. 305. 
