FORM-ALTERATIONS AND GROWTH OF CACTI. 43 



(6) The percentag-e of water in an ordinary healthy sahuaro in a dry 

 time ranges approximately from 75 per cent of the fresh weigrht in its lower 

 part to more than 90 per cent in its upper part. An individual 6 meters 

 in heig'ht absorbed and stored approximately 412 liters of water between 

 November, 1906, and March, 1907, besides the amount transpired. Trans- 

 piration from the trunk is necessarily slow, but is rapid from flowers and 

 flower-buds. The average transpiration of a fully-opened flower is at least 

 850 mg-. for an hour of full sunlig-ht. About 3.27 liters of water are stored 

 in the tissues of every 100 flowers, but even when this is added to the 

 amount transpired it is an inconsiderable quantity in comparison with the 

 entire amount stored in the trunk. None the less, the abundance of flowers 

 has been observed to be directly correlated with the amount of available 

 water in the soil. 



(7) Comparative observations show that the bisnag-a approaches the 

 sahuaro closely in its structural features and presents essentially the same 

 form of mechanical adjustment. Its habits as reg-ards water-storag-e and 

 its relations to external factors, particularl3^ to insolation, present some mod- 

 ifications, but do not differ widely from those of the sahuaro. In mode of 

 growth, however, there are important differences, and the apical gfrowth of 

 the bisnag'a, as compared with the sahuaro, is exceeding-ly slow. In like 

 manner, observation of different species of flat opuntia g:o to show that the 

 proportions of water in the body of the plant and in flower-buds are not 

 far from those of the sahuaro, and that in g'eneral the behavior of plants of 

 this g'enus, as reg'ards absorption, water-storag'e, and loss, is very similar 

 to what has been observed in the giant cactus, while in the mode of g-rowth 

 altogfether different relations prevail. None of the opuntias approach 

 the sahuaro in perfection of mechanical adjustment, in development of the 

 water-storage system, or in growth in dimension, yet they are extremely 

 well adapted to the conditions under which they are now living over a 

 far wider area than that inhabited by the sahuaro. 



Comparison of the several genera thus far studied goes to show that the 

 end attained by the highly perfected mechanical adjustment of the sahuaro 

 may be realized apparently quite as efficiently in a much simpler way. An 

 Opuntia joint may be roughly compared to a flat rubber bottle which swells 

 or shrinks according to the quantity of water which it has received or lost. 

 There is no complicated device whatever in its construction, and yet with 

 its exceedingly simple structure the Opuntia, as already stated, has spread 

 far more widely than the sahuaro and is now adapted to a far wider range 

 of external conditions. Thus it appears that, among the cacti at least, 

 the lines of descent in which simpler rather than more highly perfected 

 adjustments in relation to water-storage have been developed are at pres- 

 ent the most successful as regards survival in a great variety of external 

 conditions and extension over a wide range of territory. 



