2J)4 PHYSIOLOGICAL BOTANY. PABT II. 



ing the directions of the stems of plants. When grown 

 in a chamber which admits the light on one side 

 only, they constantly incline towards it. This has been 

 supposed to be owing to a greater decomposition of 

 carbonic acid on the side which is towards the light, 

 and a necessarily greater deposition of carbon on that 

 side than on the other. This produces a greater rigidity 

 in those parts, and consequently a curvature on the side 

 which is towards the light. This effect is produced 

 only on the young green parts of plants, and does 

 not take place in the old woody portions ; nor is it 

 observed in parasitic species, which are without the 

 means of decomposing carbonic acid. The missletoe 

 forms a most remarkable exception to the usual laws 

 which regulate the direction of the root and stem. 

 If a seed of this plant be attached to a piece of glass 

 placed over a dark surface, the radicle invariably ex- 

 tends itself in a direction opposite to the side in which 

 the light shines, from whatever quarter it may come. 

 The branches of this plant are also developed indiffer- 

 ently in all directions, without any obvious tendency 

 either upwards or towards the side from whence the 

 greatest illumination may proceed. 



(302.) Botanical Geography. We cannot dismiss 

 the physiological department of our subject, without 

 referring to that branch of it which treats of the 

 natural distribution of plants on the earth's surface in 

 other words, to " Botanical Geography." It is a fact 

 sufficiently familiar to every one, that different species 

 of plants affect peculiar situations ; some love an ex- 

 posed aspect, others prefer shady places ; some are 

 found in mountainous districts, others in plains, in 

 marshes, and even wholly submerged in lakes, or in 

 the sea. The various physical circumstances attend- 

 ing different spots in the same range of country 

 determine the "stations" in which the different spe- 

 cies of plants can grow. We know that different 

 plants require different degrees of temperature ; some 

 are calculated to live in cold or temperate climates, 

 whilst there are others which belong to the torrid zone ; 



