164 THE GEOGRAPHICAL DISTRIBUTION. 



and simply stops in consequence of the decreasing heat and 

 light of the sun, is evident from the fact, that plants which 

 are annual and herbaceous in temperate climates, be- 

 come ligneous perennials in the tropics. The castor oil 

 plant, (Ricinus cornmunis,) for example, in Pennsylvania, 

 puts forth large peltate-palmate leaves, and grows from 

 three to eight feet in height, but is destroyed by the first 

 frosts of autumn. In the happy regions within the tropics, 

 its stem is ligneous and persistent, and it grows into a 

 powerful and lofty tree. It is the same with the Euphor- 

 biacese, Labiatae, Leguminosae, Boraginaceas, Hypericacese, 

 Rubiaceae, YerbenaceaD, Polygonaceae, Composite, and a 

 host of other plants which we tread under our feet in 

 Pennsylvania ; these die down to the earth's surface and dis- 

 appear from the landscape on the approach of winter, which 

 arrests the movements of life in all the lower forms of organ- 

 ization in temperate climates. In the tropics, these very 

 plants, so herbaceous and perishable with us, take a lig- 

 neous and persistent form, and elevate themselves majes- 

 tically into the air. Excepting on the mountain summit, 

 snow never falls on any other part of the warm and sunny 

 landscape, and the traveller wanders amid the arborescent 

 forms of Legurninosse, Euphorbiaceae, Labiatae, and Bora- 

 ginaceae ; or if he be in the island of St. Helena, reposes 

 beneath the shade of forests of Solidago, Sonchus, and 

 Echium. The herbaceous and perishable annual has be- 

 come transformed into the ligneous and enduring perennial. 

 The plant whose humble growth and delicate beauty drew 

 our admiration as it grew at the foot of some tall oak or 

 lofty buttonwood, is now itself one of the noblest trees of 

 the forest. Development has gone on, and we see the 

 result of the magic influence of a continuity of warmth and 



