64 ACIDITY AND GAS INTERCHANGE IN CACTI. 



summer, following the summer rainy season during which it was produced. 

 The really essential part of the active tissue must be comprised in the mature 

 joints when they are turgid, a condition realized during the rainy seasons, 

 which are characteristic of the region around Tucson, and for a time subse- 

 quent to these periods. During the winter rains there will not be any of the 

 young joints in the case of Opuntia versicolor, since these are formed almost 

 exclusively in the summer. As will be seen later, the winter and spring 

 seasons must be a highly important period for this species of cactus, if not also 

 for others, for it seems that the bulk of the actual storing of potential energy 

 must take place during the bright days in the winter and the warmer but still 

 relatively cool days of the spring. The condition presented by the flaccid 

 joints does not, it seems, inhibit their activity, for while they are relatively 

 quiescent the gas interchange is by no means non-existent. Whether they 

 carry on much photosynthetic work is perhaps a question, for in appearance, 

 at least, the chlorophyll is certainly less in evidence than when the plants are 

 turgid. Not a small portion of the year, however, is spent in a desiccated 

 condition by the smaller cacti, which do not store the enormous quantities 

 of water found in the more bulky forms. 



It has not been possible to trace the gradual transition from the turgid to 

 the flaccid condition, for neither of the writers has been in Tucson at the proper 

 time of year, which would be in May and June or from late August to October. 

 But the assumption of a turgid condition from the flaccid state is very easy of 

 accomplishment. In as short a period as 5 days, a plant which was copi- 

 ously irrigated in 1913 increased its water-content from about 66 per cent to 

 79 per cent, or, in other words, its dry weight fell from 33 to 21 per cent. By 

 further irrigation for about two weeks the dry weight fell to about 17 per cent. 

 In the summer, after the rains have commenced, it is safe to say that within a 

 week the plants are practically fully turgid and active. 



As a matter of practical experimentation it was easily possible in the summer 

 of 1912 to carry on simultaneous determinations with flaccid, turgid, and 

 young joints. Certain plants in the neighborhood of an irrigated area were 

 already in full activity and had put forth a quantity of young shoots by the 

 middle of June. At the same time the plants in their natural condition, 

 removed from any source of water, were wholly flaccid. Not one showed 

 signs of producing young joints. 



The condition in 1913 was somewhat different. The winter had been 

 an exceptionally cold one, indeed almost a record for the region, and a great 

 deal of winter-killing had taken place; also, there had been a shortage of rain 

 during the winter months. Despite the latter circumstance and perhaps 

 because the actual mass of the plant had been so greatly diminished by the parts 

 lost by freezing, plants wholly removed from any source of irrigation had put 

 forth new shoots, although the mature joints were shriveled and dry. There 

 could be no mistake that these shoots were new growth, for they were all 

 provided with leaves. They showed a dry weight of about 20 per cent, which 

 is considerably greater than that usually found in young tissue, which it 

 will be remembered is about 15 per cent. Indeed, the dry weight of these 

 joints closely approximated that w r hich had been determined in the previous 

 summer as the average for the mature-turgid joints. Nevertheless they 

 behaved essentially as young joints are wont to and were putting forth an 



