176 BOTANY 



at which it is operative in calling forth the most active manifestation 

 of a definite vital phenomenon is termed the OPTIMUM. Minimum, 

 optimum, and maximum are not, however, points of fixed value for 

 the organism, but vary, on the one hand, with the duration of 

 exposure ; on the other, with the sum of the other conditions. For 

 the different vital processes of the same plant, and also for those 

 of distinct plants, these so-called CARDINAL POINTS are generally 

 different. 



The cardinal points for the vegetation of cold climates are on the average much 

 lower than those of plants inhabiting temperate or tropical countries. The dis- 

 tribution of plants over the surface of the earth is thus to a considerable extent 

 dependent on their cardinal points. The highest are not, however, possessed by 

 tropical plants, but by small Algae and Bacteria which inhabit hot springs, the 

 water of whicli has a temperature of 70-80 C. The albumen of a hen's egg is 

 quickly coagulated by the water in which these plants find their suitable habitat, 



Fio. 172. Protonenia of Schistostega osmundacea. The lens-shaped cells concentrate 

 the feeble light available in the habitat of the plant upon the chloroplasts. 

 (Cf. Fig. 214.) 



while some thermogenic Bacteria can raise their own temperature to 70 C., and 

 even higher. 



Some plants flourish best when exposed to bright sunlight, while the shade- 

 loving plants only attain their perfect development in a subdued light, such as 

 that of a forest, or like Schistostega in crevices and caves (Fig. 172). Not only 

 does the intensity of the required illumination differ for different species of plants, 

 and also for individuals of the same species, but the optimum intensity may change 

 as the development of the individual plant proceeds. 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. Young plants require to 

 be protected by specially planted shade -producing trees (Albizzia moluccana, 

 A. stipitata, etc.). 



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

 plants become frozen and generally die. 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 the 

 tissues does not of itself occasion death. The formation of ice always begins in 

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

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



