100 ENVIRONMENTAL CONDITIONS. 



The generalization just mentioned is very greatly complicated, as 

 has been indicated, by the variations and fluctuations in the internal 

 physiological conditions as development proceeds. The limits for 

 life are often very different in the various developmental phases of 

 the same form. Thus, mature seeds of many temperate and boreal 

 annuals show temperature minima far below the freezing-point of 

 water, while the vegetative phases of the same forms may succumb to 

 the first frost. Winter buds of northern deciduous trees possess high 

 powers of resistance to low temperatures, while summer buds of the 

 same plants may not bear temperatures as low as the centigrade zero. 



Besides the variations in limits of growth and life in different phases 

 of the same form, it must be remembered that, in any phase or at 

 any time in the life of the organism, there are a number of different 

 processes going on, such as photosynthesis, respiration, digestion, 

 secretion, and the like, each of which has its physiological limits, and 

 the limits for one process are frequently not at all the same as for 

 another. In general, the pair of limits that characterize these simpler 

 processes, which together make up the vegetative or reproductive activ- 

 ities of a plant, are much less widely divergent than those for the mere 

 retention of vitality itself. By retention of vitality we probably mean 

 the occurrence of the life processes at their lowest intensity, an inten- 

 sity that is just sufficient to maintain life, though this expression may 

 be taken in a general way to denote simply the power of initiating the 

 various more vigorous and obvious processes when conditions become 

 right. Seeds retain their power to start the germination processes 

 for long periods of time under conditions that preclude germination 

 itself. Again, with increasing scarcity of water or lowering of temper- 

 ature, the growth processes of all plants are sooner or later brought to 

 a standstill, long before death ensues. Moisture conditions that are 

 optimal for vegetative growth frequently prevent the production of 

 fruit, so that gardeners make it a practice, with the coming of the 

 flowering-time in many plants, to diminish the water-supply. 



That the factor of time enters into the determination of physio- 

 logical limits is obvious. Many plants are able to survive a short 

 period with the soil about their roots in a saturated condition, but 

 succumb to a longer period of exposure to a saturated soil. Numerous 

 forms retain their vitality through long periods of drought, when the 

 soil is nearly air-dry, but if the dry period is sufficiently prolonged 

 death is the inevitable result. As has been mentioned, general 

 growth is not noticeably affected by the regularly recurring nocturnal 

 period without illumination, but etiolation becomes marked, and 

 various other pathological conditions are induced when ordinary 

 plants are kept in continuous darkness for but a few days. In phys- 

 ical terms this means merely that the effects of any set of external 

 conditions upon the plant are always cumulative and are exaggerated, 

 in one way or another, with the lapse of time. 



