BIOLOGY AND LIFE HISTORY. 15 



of which those formed in the wood zone of the host become applied 

 to the water-carrying cells of the host, thus establishing an efficient 

 means of conducting water and mineral nutrients from the host into 

 the parasite. The mistletoe seedling is now virtually a tiny bud 

 graft (fig. 3). 



Having thus established its connection with the source of some of 

 its food supply, the upper end of the hypocotyl with the minute 

 cotyledons gradually withdraws from the inclosing and partly di- 

 gested endosperm, and becoming erect the cotyledons slowly expand 

 as the first pair of green leaves. Perhaps this is as far as develop- 

 ment goes during the first season. In some species of mistletoe the 

 cotvledons remain covered by the seed jacket and endosperm (if 

 there is any) during all of the first season. Apparently the progress 

 of development at this period depends upon weather conditions. In 

 a dry climate these are apt to be such as to interrupt repeatedly the 

 progress of germination and of becoming established. Existence 

 becomes largely a question of endurance during this period, and it is 

 probable that hi a season of unusiud warmth and humidity the seed- 

 ling progresses hi its development farther than merely to expand the 

 cotyledons: whereas, in an unfavorable season, if the seedling sur- 

 vives at all it may approach the winter with germination, in the popu- 

 lar sense, still incomplete. This seems to be a reasonable interpreta- 

 tion of facts and conditions thus far observed in the field, and it points 

 with renewed emphasis to the peculiarly resistant qualities of the 

 mistletoe at this period of its existence. In spite of these qualities, 

 however, the great majority of mistletoe seedlings perish, or the 

 seeds never germinate at all in the face of such extreme arid condi- 

 tions as frequently prevail. 



It should be noted that the mistletoe in establishing itself as pre- 

 viously described, with its primary sinker in contact with the wood 

 cells, has the problem of adjusting itself to the season's growth in 

 thickness. This it does by maintaining a zone of embryonic tissue 

 in the sinker corresponding to the cambium zone of the host. 



Some time after the seedling has established itself as described, 

 probably not until the second season's growth, there arise from the 

 primary sinker in the zone of soft bark, or cortex, lateral outgrowths 

 called cortical roots, or cortical haustoria, which spread along and 

 around the host stem in this cortex zone, extending farther and far- 

 ther from the original point of penetration (fig. 4). At intervals 

 from the side of the cortical roots nearest the wood zone more sinkers 

 arise and penetrate along the line of least resistance (medullary rays) 

 into the wood, where they develop a connection with the water-carry- 

 ing vessels as the primary sinker did. These cortical roots persist 

 and increase in thickness; indeed, it appears to be the thickening of 

 these at their junction with the parent axis which gives the enlarging 



1G6 



