DISTRIBUTION AND HARMFUL OCCURRENCE. 9 



relatively more abundant than it is eastward. It is a curious fact, 

 whose explanation is not very obvious, that mistletoe is more varied 

 in form and relatively more abundant in the arid districts of the 

 Southwest (e. g., portions of Xew Mexico and southern California) 

 than it is in the Gulf States east of the ninety-sixth meridian. It is 

 not that the growth of mistletoe as a shrub is more vigorous than in 

 the eastern belt; as a matter of fact, in the more humid climate it 

 makes a most luxuriant and symmetrical growth, but fewer of the 

 trees relatively are infected and these evidently in more restricted 

 localities. In river or creek bottoms or in swamps one sees the taller 

 trees with bunches of mistletoe far up on the remote branches. In 

 middle Texas, on the contrary, mistletoe is by no means confined to 

 bottom-land trees, but it infests those of upland prairies also. Stunted 

 native growths and transplanted trees are especially apt to be infected, 

 and not merely by isolated bunches of mistletoe on remote branches, 

 but throughout the tree on old as well as young branches, and not 

 infrequently upon the main trunk itself, so that the whole tree is in- 

 fected, weakened and disfigured, and finally killed. (PL I, fig. 1.) 

 Thus it comes about that just in the region where trees in perfection 

 are especially difficult to find they are more than elsewhere subject 

 to harmful infection by this parasite. The explanation may be 

 suggested that mistletoe, like a good many other plants of arid situa- 

 tions, requires much sunlight for its best growth and especially for 

 the development of flowers, and thereby of numerous and vigorous 

 seeds, and is at a disadvantage in competing with the heavy shade- 

 casting foliage of forests in humid climates. The necessity for light 

 might explain why in bottom-land forests of the East mistletoe is 

 confined to the highest branches of the tallest trees (as shown by 

 observations made in the case of bottom-land timber in parts of 

 Arkansas, southeastern Oklahoma, and northeastern Texas), and 

 how with increasing intensity of sunlight and the more meager foliage 

 and open stand of trees incident to the drier climate of the Southwest 

 mistletoe is enabled to spread over the entire tree. 



That more arid environmental conditions have acted in the nature 

 of a stimulus to mistletoe in its growth and reproduction may be 

 alleged from the fact that in the arid Southwest there is much more 

 variation in the habit of the plant than there is in the Atlantic and 

 Gulf forest regions. Thus between middle Texas and southern Cali- 

 fornia there are several distinct types or varieties of Plxoradendron 

 flavescens (macro phyllum, villosum, orbiculatum, and pubescens), 

 besides several more distinct species, notably Plxoradendron calif orni- 



a Exceptions to this statement are not unusual. Tims, at Houston. Tex., at various 

 points in Louisiana, and also in some localities in Mississippi and Tennessee, mistletoe 

 has been reported as harmfully abundant in isolated trees or clumps of trees in door- 

 yards and parks. 



12579— Bui. 166—10 2 



