54 THE ORIGINATION OF PARASITISM. 



that a parasite must withdraw material from its host in accordance with 

 the osmotic power of its juices. 



The relative concentration or freezing-point of the sap of two plants 

 would, therefore, be a factor in the determination of their meeting: in a 

 parasitic union. This is undoubtedly an ultimate limiting- condition act- 

 ing: as a barrier which all but closes the narrow gateway through which 

 two seed-plants must meet if they are to assume the relations of host and 

 dependent. It will now be in order to discuss the results of experimental 

 arrang-ements of plants with respect to these operative factors and limit- 

 ing - conditions. 



The material employed included Echinocactus (the melon cactus), Car- 

 negiea (the tree cactus or sahuaro), and various species of Opiuitia as en- 

 forced hosts, and two Mexican grapes or Cissus, Tradescantia, Agave, and 

 various cylindrical and flat-stemmed opuntias as "xeno-parasites," as it is 

 proposed to designate them. 



Sections of stems a few inches in length were prepared and allowed to 

 reg-enerate the wounded surfaces, with the formation of preliminary root- 

 extrusions on some cases. Cavities were then made in the bodies of the 

 hosts, and the bases of xeno-parasites inserted. In some cases the turgid 

 tissues rich in mucilag-e would close around the inserted slip in such man- 

 ner as to seal the preparation effectively, while in other cases gelatine, 

 wax, and plaster were employed for the purpose of holding- the would-be 

 parasite in place and preventing- too ready access of air. Many hundreds 

 of preparations were made, some of which were so successful that the xeno- 

 parasite has successfully maintained itself for two years in connection with 

 the host, making- new internodes and leaves in a characteristic manner. 

 The length of time over which the work has been carried permits a fair 

 analysis of the adaptive adjustments which have been called into existence 

 experimentally. 



First, it is to be seen by a casual inspection of the experimental results that 

 the osmotic activity of the sap and its variation throug-hout the seasons is 

 a matter of the greatest importance in the approach of two forms to the 

 position of host and dependent. Cissus, with an osmotic activity of 11.34 

 atmospheres (ash in 100 c.c, 1.39) was successfully parasitic on Opuntia 

 blakeana at 8.88 atmospheres (ash, 1.15). but less so on Ju/iinocactus at 

 5.72 atmospheres (ash, 1.20), and not all on Carnegiea at 6.78 atmospheres 

 (ash, 1.00), the failures in the two last-named being: due to other condi- 

 tions than absorbent capacity. Opuntia blakeana at 8.88 atmospheres 

 (ash, 1.15) was successfully parasitic on Echinocactus at 5.72 atmospheres 

 (ash, 1.20). Opuntia versicolor at 11.98 atmospheres (ash 2.10) main- 

 tained itself upon O. blakeana at 8.88 atmospheres (ash, 1.15), upon 

 Carnegiea at 6.78 atmospheres (ash, 1.00), and Echinocactus at 5.72 atmos- 

 pheres (ash, 1.20). Agave at 10.84 atmospheres (ash, 1.096) was sne- 

 cessful in maintaining- existence on Opuntia blakeana at 8.88 atmospheres 



