14 



THE MISTLETOE PEST IN THE SOUTHWEST. 



appears capable of repeating this behavior of alternate activity and 

 dormancy throughout the germination period and until the seed- 

 ling is fully established, and therefore less influenced by sudden 

 fluctuations of moisture and dryness. In short, the seed in its 

 germination phase is a well-adapted drought plant or xerophyte. 



GERMINATION AND ESTABLISHMENT OF THE SEEDLING. 



The first apparent movement in germination consists in the elon- 

 gation of the axis (hypocotyl) of the embryo, whereby the root 

 tip is thrust beyond the beak of the seed jacket, thus becoming 

 directly exposed to the air and sunlight. Upon becoming exposed 

 the axis bends so as to bring the blunt end of it into contact with 

 the underlying branch. This prompt bending in its growth appears 

 to indicate a sensitiveness of the tip toward the branch, but it has 



been shown that it is on account of its 

 sensitiveness to light that the axis 

 bends, and the bending has for its 

 object the avoidance of too intense 

 illumination. This, however, results 

 in bringing the root tip of the embryo 

 against the branch. The root end of 

 the embryo is blunt or even knob- 

 shaped while still within the seed 

 jacket, and as it emerges and turns 

 toward the branch the end further 

 enlarges until it becomes promi- 

 nently knob-formed even before it 

 comes into contact with any under- 

 lying object (fig. 2). When, how- 

 ever, it touches the branch, the knob 

 becomes broader and is flattened on 

 the contact side, as when a plastic ob- 

 ject is pressed firmly against a hard surface. Thus it behaves like an 

 adhering disk or sucker, such as one observes in the Boston ivy, where 

 the tips of the clinging tendrils flatten against the wall. At the center 

 of the flattened disk the cells continue to push forward, with the 

 result that a point of tissue, the primary sinker, bores its way, or, 

 perhaps more accurately, dissolves its way, into the tissues of the 

 branch (fig. 3), for the cells of this puncturing point are able to 

 secrete a substance (enzyme) capable of dissolving the walls of cells 

 lying in its path. Thus it really dissolves and absorbs the tissue 

 of the host. This primary sinker pushes downward through the 

 soft bark zone (cortex, phloem, and cambium) until it reaches the 

 younger wood cells. Meanwhile an axial strand of cells in the pri- 

 mary sinker becomes differentiated into water-transporting tracheids, 



166 



Fig. 3. — Diagram showing the method of pene- 

 trating a branch by a mistletoe seedling: a, 

 Corky covering of branch; b, cortex zone; c, 

 bast fibers; d, cambium or growth ring; e, 

 wood zone; /, sinker of mistletoe penetrating 

 to wood zone ; g, disk; h , body of mistletoe seed. 



