106 ENVIRONMENTAL CONDITIONS. 



Plant protoplasm may be said to pass through alternating phases or 

 stages of diastole and of systole. In the condition of diastole it is very 

 active, increasing rapidly in amount and extending and altering its 

 configuration to a remarkable degree. These changes are due directly 

 to numerous physico-chemical processes and energy transformations 

 which, during the period of this phase, are quite violently active. In 

 the condition of systole, on the other hand, the various characteristic 

 life processes are at a low ebb, some of them being apparently alto- 

 gether abated. The mass and extent of the protoplasm falls off more 

 or less markedly, in the death of many parts which were previously 

 the seats of vigorous activity, and in many cases the whole organism 

 practically fails to exhibit any form of hfe at all. This rhythmic 

 pulsation is of course immediately due to internal conditions, but the 

 latter are, in turn, to be causally related either to changes in the envi- 

 ronment or to effects of a constant environment summed or integrated 

 by the organism. 



While the point of view just suggested is not at all new, and has 

 been of great service to some students of heredity, we are not aware 

 that it has been resorted to in studies of the influence of external con- 

 ditions upon the maintenance of plant population in a distributional 

 sense. It seems to promise such utiUty as a logical tool that we venture, 

 in the following paragraphs, to outline the behavior of some of the 

 main plant types in terms of this conception. 



Attention may first be directed to the case of a perennial which 

 propagates vegetatively, omitting for the present any question as to 

 whether seeds are produced. An excellent example of this is seen in 

 several forms of much-branched cylindrical opuntias, as the various 

 ''cholla" cactus-forms of the North American Southwest. By the 

 action of various agencies, such as wind and animals, short branches 

 are easily broken off from these plants, and are widely distributed by 

 the operation of the same agencies and by that of flood-water. Under 

 favorable conditions these fragments possess the property of taking 

 root and forming new plants. Such may be regarded as the simplest 

 form of species maintenance, and it is of course the rule among the 

 lowest forms of plants. We have here to do with the mature plant at 

 all times, the only complication in vegetative phases lying in the fact 

 that roots are produced and proceed with their characteristic activ- 

 ities under the peculiar conditions offered by the dispersal of the joints. 

 With the coming of each favorable season, mainly defined by condi- 

 tions of moisture and temperature, growth in size occurs in numerous 

 branches, some of the latter being portions of larger plants, while others 

 lie singly upon the ground. By the action of adverse conditions many 

 branches are destroyed, but enough survive apparently to maintain 

 the status of the species in the vegetation of its area. During the vear 

 great fluctuations occur in the quality and intensity of the environ- 

 mental factors; periods of extreme drought and heat, periods of drought 

 and cold, periods of abundant moisture and either high or low tem- 



