36 Plant Life and Evolution 



structures in the highest plants ; but it is perfectly 

 clear that the stem and leaves in the seaweed and 

 the seed-plant have no genetic connection, but have 

 arisen quite independently in the course of evolu- 

 tion in the very widely separated plants, in response 

 to the same needs. We meet with the same phe- 

 nomenon again among the mosses, where there are 

 developed perfect leaves of a type quite different 

 from those of either the seaweed or the flowering 

 plant. 



The extraordinary variety of leaf structures 

 found among the flowering plants can usually be 

 correlated with special adaptations to light condi- 

 tions, and the adaptability of individual plants in 

 this respect is extraordinarily great. A further 

 discussion of the relations of plants to light must 

 be left for another chapter. 



Range of Temperature Suitable for Plant Growth. 

 An indispensable condition for the manifestation 

 of life in any organism is a suitable temperature. 

 However, there may be a great deal of difference 

 shown by different plants in the range of tempera- 

 tures which they can endure. As a rule all mani- 

 festations of life cease in plants when the surround- 

 ing medium is cooled to the freezing point of fresh 

 water (o C), but some seaweeds thrive in water 

 which may fall below this temperature and seldom 

 has its temperature much above o C. Such cold 

 water algae are quickly killed by a temperature 

 only a few degrees above the freezing point, while 



