3] PHYSIOLOGICAL REACTIONS 79 



environmental conditions. On a plant's capacity for them may 

 depend its geographical range. 



The production of reproductive bodies involves various physio- 

 logical activities that are closely correlated and indeed wonderfully 

 integrated, yet may be affected by environmental conditions in a 

 unique way. Although some trees may live for more than 2,000 

 years, there is no known instance of life being really permanent in 

 any individual. So in order to persist a plant must reproduce, and 

 any condition which prevents it from doing so in a particular area 

 will preclude that area from its normal range (that is, in the absence 

 of persistent immigration). Many conditions — climatic, nutritional 

 or otherwise — can and frequently do prevent the normal reproduction 

 of certain plants, so limiting the geographical areas they occupy. 

 Some plants circumvent this either by separating off parts of their 

 bodies for ' vegetative ' reproduction or by the development of 

 special organs for the same purpose, thereby enabling themselves 

 to persist in areas where seed etc. cannot be produced. This is 

 true of many plants living under extreme conditions, for example 

 in the Arctic. 



Although on land there is almost everpvhere sufficient light to 

 enable plants to grow, the effect of light-climate on their reproductive 

 processes affords another instance of range-limitation. For many 

 plants require a day-length within particular limits before they can 

 flower successfully, and, in latitudes where the length of day during 

 their flowering period is outside these limits, are unable to reproduce 

 sexually. This appears to be one reason why many southern 

 species fail to flower in the north, and vice versa. However, such 

 reactions are by no means immutable, but tend to vary with other 

 conditions, and may also be changed by treatment with certain 

 chemicals. 



The ranges of particular species may be limited by chemical 

 ' antagonism ' {i.e. active opposition to growth, etc.), nutritional 

 conditions, and other factors bound up with the soil. Familiar 

 instances are afforded by some plants which require much ' lime ' 

 (actually, calcium carbonate) in the soil, and others which avoid it. 

 Examples of the former category are Yellow Mountain Saxifrage 

 {Saxifraga aizoides agg.) and Salad Burnet {Poterium sanguisorba), 

 and, of the latter, most Heaths (Ericaceae). Often the merest trace 

 of a particular compound or element, such as boron, can have a 

 profound effect in encouraging or precluding particular species. 

 Deficiency diseases, due to lack or insufficiency of particular 



