10 THE MISTLETOE PEST IN THE SOUTHWEST. 



cum, which infests many species of desert trees and shrubs, and Pho- 

 radendron bolleanum and P. juniperum, which last two are especially 

 noteworthy as being parasitic upon coniferous trees, especially desert 

 junipers. 



BIOLOGY AND LIFE HISTORY. 



A brief account of the life history and of the habits of nutrition of 

 mistletoe will assist in making clear the reason for its harmfulness 

 to trees. The plant is a parasite. It fastens itself upon its host, the 

 tree, penetrates its tissues, and draws nourishment from it, deforming 

 it and sapping its vitality. Yet the mistletoe is a green, leafy plant ; 

 that is, it possesses the pigment chlorophyll, which gives the green 

 color to normal vegetation. Some of the tropical species of mistletoe 

 bear leaves as large as those of an American elm. The presence of 

 green leaves indicates that the mistletoe has the power, which inde- 

 pendent green plants everywhere possess, of constructing organic 

 foodstuffs, such as starch, out of inorganic compounds (carbon dioxid 

 and water), utilizing sunlight as the source of energy in the process. 

 It is therefore only partly a parasite so far as dependence upon a host 

 for food is concerned, but apparently none the less harmful on that 

 account. . It secures from its host apparently only what the normal, 

 shrubby plant derives from the soil, namely, water and certain 

 necessary mineral constituents. Imagine a grapevine or trumpet 

 creeper, while retaining its foliage, to sever connection with the soil 

 and to thrust root-like outgrowths into the body of the tree to which 

 it clings, in order to absorb from the tree what before it absorbed 

 from the soil. This would represent the relation which mistletoe 

 sustains to its host. 



In this connection it is instructive to observe that the mistletoe 

 family, viewed as a whole, shows a progressive development of para- 

 sitism. Thus at one end of the series stands the Australian genus 

 Nuytsia, whose single species is a nonparasitic tree. At the opposite 

 extreme is the degenerate, absolute parasite Phrygilanthus aphyllus, 

 parasitic upon a cactus of the genus Cereus in Chile. This plant is 

 said to possess neither cotyledons nor foliage leaves, nor does it 

 develop the vigorous shrubby habit characteristic of mistletoes gen- 

 erally. The more familiar mistletoes are sometimes called "half 

 parasites," but they also show great variation in habit from the very 

 broad-leaved forms above mentioned down to those which are yellow- 

 green and quite leafless; e. g., Phoradendron juniperum and Arceuiho- 

 bium pusillum, the latter of which barely emerges from its host. 



In the general sequence of events the life history of mistletoe is 

 just like that of any flowering woody plant; for example, the hack- 

 berry, upon which it preys. It bears flowers; in due time the berries 

 follow, each with its inclosed seed; the berries are deposited by birds 



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