^34 baker's north VoricshireI. 



exercised opens out a wide field for research and consideration. 

 A plant is not a mere machine, like a thermometer, but a living 

 organism, and in considering the question we must take care to 

 remember that such is the case. Conditions of life and biological 

 phenomena we have need to bear constantly in mind, and here 

 as elsewhere, the mysterious principle of vitality constantly inter- 

 feres to limit the application of our generalisations. Especially 

 have we to remember the differences in respect of duration which 

 plants present. An Annual plant normally grows up to perfection, 

 and produces flower and seed during the same year in which 

 the seed that produces it was sown. A Biennial plant produces 

 only stems and leaves the first year, flowers and fruit the second, 

 and like the Annual, after one flowering and fruiting dies away. 

 A Perennial plant lasts for an indefinite number of years, yielding 

 flowers and fruit from the same root an indefinite number of 

 times. So that in any locality the plants of these different 

 categories in respect of duration are exposed in a very different 

 manner to the range of variation which its temperature presents. 

 Trees and shrubs, especially evergreens, are exposed more or 

 •less to the temperature of all the seasons of the year ; herbaceous 

 perennials and biennials, as a general rule, less so to the colds 

 of Winter, especially in those countries where, as in ours, the 

 cold almost suspends vegetation for a period, and where the 

 ground is often overspread for a length of time with a covering 

 of snow ; whilst annuals mostly grow up to perfection and perish 

 during, in our climate, a few months of the warmer part of 

 the year. 



Different species attain perfection at different seasons of the 

 year, some earlier and some later, some in Spring, some in 

 Summer, others in Autumn, some having a wide range of flower- 

 ing and fruiting time, others opening out their flowers in any 

 locality, at a particular time, year after year, with great regularity. 

 No phanerogamous plant can develop itself below the freezing 

 point, and in different plants the sap begins to circulate at very 

 different degrees of the thermometer. The seeds of Capsella 



