6 BOTANY 



While it is impossible, then, to make any absolute distinctions 

 between animals and plants, we may say that in general, the most 

 marked characters of typical plants, as distinguished from animals, 

 are (1) the presence of chlorophyll, and the accompanying power of 

 photo-synthesis ; (2) the presence of a cellulose membrane about the 

 cells ; (3) the absence of locomotion in the plant-body. 



Conditions of Plant-life l 



Since all animals are directly or indirectly dependent on plants for 

 food, it follows that wherever animal life exists, plants can also grow. 

 Green plants, of course, can only thrive where a certain amount of 

 sunlight is present, since photo-synthesis, or the assimilation of car- 

 bon-dioxide, is dependent on light. The amount of light necessary is 

 extremely various. Thus, many Seaweeds grow in water so deep as 

 to exclude much of the light, and some Ferns and Mosses live in 

 dimly lighted caves, or flourish in the twilight of dense forests ; while 

 Cacti and Palms endure the full blaze of an unclouded tropical sun. 

 We shall consider later some of the ways in which plants adapt 

 themselves to the varying amount of light. 



Temperature. There is a certain range of temperature within 

 which the vital functions of plants are active. As might be expected, 

 this range is different for different plants. Some plants flourish at 

 a temperature close to the freezing point of water, and may be frozen 

 while actively growing, without injury. Others are quickly killed 

 by a temperature considerably above the freezing point, while they 

 thrive best at a high temperature which would almost instantly 

 destroy a Seaweed accustomed to the cold water of the northern 

 Ocean, or an Alga growing in an icy mountain stream. 



It is among the lower plants, and the dried resting structures, like 

 seeds and spores of the higher ones, that the greatest powers of 

 resistance to extremes of temperature are found. Even in their 

 active condition, many Bacteria can endure an extraordinary range 

 of temperature, but it is the resting stages, or spores of these, as well 

 as the seeds and spores of the higher plants, which are most resist- 

 ant, especially to extremely low temperatures. 



The presence of water in the cells makes them far less resistant 

 to both high and low temperatures. Especially sensitive are plants 

 like many Seaweeds, which grow in cold water which varies but little 

 in temperature throughout the year. These plants are destitute of 

 the protective structures which have been developed by land plants. 



Water in Plants. All manifestations of life are bound up with 

 the presence of water. Without it the protoplasm cannot act ; and 



l The special physiology will he treated more fully in later chapters. 



