i62 THE BIOLOGY OF FLOWERING PLANTS 



and in autumn, when no rain falls. They transpire con- 

 tinuously, though the amount seems to bear little relation 

 to the weight or surface of the plant. An Echinocactus, a 

 globular type, for instance, weighing 42 kg., was taken up 

 and kept in the laboratory for 16 months without water. 

 It transpired at the rate of from i to 29 grm. daily, losing in 

 all 4 kg. of water before it died. Another plant, weighing 

 17 kg., lost 5 kg. between November and May ; it was 

 then taken up and placed in the soil, where it made up its 

 loss and showed growth, weighing 20*5 kg. in October. 

 In the follov>^ing May it had fallen to 13*5 kg., but was still 

 quite healthy. Here we have a plant losing 30 per cent, of 

 its weight, or 40 per cent, of its water content, without injury. 

 MacDougal states that Carnegeia, a columnar type, may 

 lose as much as 63 per cent, of its water content without 

 permanent injury. The immense quantities of water stored 

 in these plants may be judged from the estimate that a 

 Carnegeia 6 ft. high may contain over 30 gallons of water ; 

 while a full-grown branched individual, with a maximum 

 diameter of 2 ft. and a height of 40 ft,, may lose and gain, 

 in the course of a season, a matter of 100 gallons. The loss 

 of water is accompanied by very marked contraction. 



For succulents of the Sedum or Mesembryanthemum 

 types the balance is probably much smaller, though for 

 these exact data are wanting. Plants with epidermal 

 storage tissue may lose 10 per cent, of their water before 

 flaccidity sets in. 



Where a specially diiferentiated water storage tissue 

 exists, it can be of service only if the assimilating cells can 

 remove water from it, that is, if they have a higher osmotic 

 pressure. Observation has shown this to be the case. The 

 water tissue of Peperomia collapses while the chlorophyll 

 containing cells are still turgid. Haberlandt and Schimper 

 observed that in Rhtzophora mucronata, one of the man- 

 groves, the younger leaves remained fresh for several days 

 after the older leaves had wilted. The water tissues of 

 these leaves increases from one-half to two-thirds the 

 thickness of the leaf as this matures. Here we have not only 



