164 BOTANY 



exposed to bright sunlight, while the shade-loving plants only attain 

 their perfect development in a subdued light, such as that of a forest. 

 Not only does the intensity of the required illumination differ for 

 different species of plants and also for individuals of the same species, 

 but it may be inconstant even for the same plant. Shade is absolutely 

 essential for many tropical plants in a young state, although at a later 

 age they can endure and may even require the full light of the tropical 

 sun. 



On exposure to a low temperature, about the freezing point of 

 water, most plants become frozen and die, generally. Very sensitive 

 plants may even become frozen at a temperature considerably above 

 zero, before ice has been formed in their tissues. In the case of other 

 plants the internal formation of ice in their tissues does not of itself 

 occasion death. The formation of ice always begins in the inter- 

 cellular spaces and not within the cells. Its continued formation is 

 accompanied by an increasing concentration of the cell sap ; as a 

 consequence of this ice first begins to form in plants at a temperature 

 below zero, and only gradually increases in case of a greater reduc- 

 tion of temperature. 



I. The Stability of the Plant Body 



One of the most important and essential physical attributes of 

 a plant is its rigidity. Without that quality plants could retain 

 no enduring form. The capacity to return, by their own independent 

 movement, to favourable positions from which they may have been 

 forcibly disturbed by external influences, is, in trees and shrubs, and 

 also in the more rigid herbs, restricted to the extreme tips of the 

 growing stems. 



How great are the demands made upon the stability of plants 

 will be at once apparent from a consideration of a rye haulm ; for 

 although it is composed of hundreds of thousands of small chambers 

 or cells, and has a height of 1500 mm., it is at its base scarcely 3 mm. 

 in diameter. The thin stems of reeds reach a height of 3000 mm. with 

 a base of only 15 mm. diameter. The height of the reed exceeds by 

 two hundred times, and that of the rye haulm by five hundred times, 

 the diameter of the base. In comparison with these proportions our 

 highest and most slender buildings, such as tall chimneys, are 

 extremely thick structures ; in them the height is only from twelve to 

 seventeen times the diameter of the base. In addition, moreover, 

 to the great disproportion between the height and diameter of 

 plants, they are often surmounted by a heavy weight at the summit ; 

 the rye straw must sustain the burden of its ears of grain, the 

 slender Palm the heavy and wind-swayed leaves, which in Lodoicea 

 Sechellarum have a length of 7 m. and a breadth of 3-4 m., while 

 in the Palm jRaphia taedigera the leaves reach a size of 20 m. in 



