66 LIFE : OUTLINES OF GENERAL BIOLOGY 



spaces in the plant's cellular architecture will lower the actual 

 freezing-point. 



But suppose the plant does freeze, what is it that kills? The old 

 answer, that there is bursting and tearing of tissues, does not seem 

 to be generally true. The widespread belief that it is the thawing, 

 not the freezing, that kills is not warranted except in a few cases. 

 There is more to be said for the view that it is the withdrawal of 

 water to form ice-crystals that is fundamentally fatal. It may 

 fatally damage the molecular architecture and bring about chemical 

 disintegration; or it may make the cell-sap so concentrated that it 

 becomes poisonous; or it may be that different plants have different 

 temperature minima for their metabolism and that when they 

 stop for a long time they cannot begin again. Hut in any case there is 

 notable individuality, for one kind of plant dies before the freezing- 

 point is reached, while another can survive being frozen stiff for 

 weeks or even months. 



RHYTHM IN LIFE 



A torrential stream gives one a vivid impression of continuity, but 

 the waves of the incoming tide illustrate rhythm. Using the 

 word witlely, we mean by rhythm the regular recurrence of certain 

 changes or events, but there are very, simple rhythms like day and 

 night which hardly deserve to be called more than alternations, 

 and there are complicated rhythms like the tides. Moreover, for 

 man's senses the term rhythm is not very appropriate unless the 

 changes or events come in rapid succession, as in a melody or in 

 verses. When there is a long interval it is more useful to speak of a 

 "cycle", as in the case of sun-spots, which rise to a maximum of 

 activity every eleven years. \Ne mean by a rhythm a regularly 

 punctuated discontinuity, rising from the extreme simplicity of 

 see-saw and ding-dong to the complications of music and of growth. 

 Sometimes what seems at first sight a very simi)Ic rhythm, like the 

 beating of the heart, is found to be complicated when studied in 

 detail, for the heart does not work with the monotonous regularity 

 of a ix^ndulum. And everyone knows that besides the alternation of 

 high tide and low tide twice in the twenty-four hours, there are the 

 major rhythms of spring tides and neap tides, according as the 

 sun and moon are pulling the ocean in the same direction, at new 

 moon and full moon, or inop}x)site directions, at the quarter-moons. 

 Similarly the continual waxing and waning in the activity of bodily 

 organs, such as liver and kidneys, is also affected by the larger 

 rhythms of the seasons— in some animals very markedly. Often we 

 may think of an organic rhythm as like a wavy line on which there 

 are minor undulations both at the crest and in the trough. 



