Cannon : Anatomy of Phoradendron villosum Nutt. 387 



4. Some Biological Considerations 



In concluding this sketch of the anatomy of the mistletoe it 

 need hardly be pointed out that, whatever may be the nature of 

 the stimuli, varying pressures or what not, which bring about the 

 characteristic structure of the mistletoe, it is for the most part 

 typical of xerophytes in general. It is to be pointed out however, 

 that the extremely heavy walls of the parenchyma is a feature 

 found in many leaves, and stems of herbaceous species, which live 

 through one or more winters, or inclement seasons. Similar struc- 

 tures are to be seen in coniferous leaves, as well as in leaves of 

 some orchids such as Aplectrum. It is also to be said that such 

 strength of wall would give rigidity to the stem during seasons in 

 which turgescence in these cells is an impossible condition. 



In the consideration of the growth of the leaves it was shown 

 that they did not reach the normal thickness until the second sea- 

 son, and the mature size was not attained until an indefinite time 

 after that. The youngest leaves are illuminated from one side, the 

 ventral, only, and Fig. 3 shows very imperfectly that there is a 

 corresponding difference in the time of development of the two 

 parts. The portion toward the light appears to have a structure 

 somewhat more fully differentiated than the opposite side. In the 

 mature leaf both surfaces are equally exposed to the light, and 

 show equal development. 



Among the many questions which come up for solution in a 

 study of this kind, and which belong to the purely biological side 

 of the general problem, may be counted those which concern the 

 nature of the association of the mistletoe and the oak. Is the 

 relationship between the two entirely of advantage to the parasite, 

 or does the oak also derive benefit from it as is apparently the 

 case in Viscitm album ? * It is clear that at the season of the year 

 when the oak leaves have fallen, and when the plant no longer 

 makes food, the mistletoe might be called upon to give up a por- 

 tion of the food, which it was manufacturing, to the support of the 

 host. As far as I could learn, however, such is never the case. 

 Material collected at such a time was examined and it was found 



* Gaston Bonnier. Sur r assimilation des plantes parasites a chlorophyll, Comptes 

 Rendus, Paris, 113 : 1074-1076. 



