Febeuaky 8, 1918] 



SCIENCE 



143 



or smothering: under its rank foliage every 

 green thing that stands in its way. It is no 

 uncommon thing to see whole acres of haw 

 thickets and other shrubby growth enveloped 

 in its deadly meshes, and destined to slow ex- 

 termination by this ruthless invader. Among 

 its victims I have seen a remarkably fine 

 specimen of " tree haw " {Cratoegus viridis) 4 

 dm. in diameter, 12 m., more or less, in height, 

 and about the same in spread of crown, re- 

 duced to little more than a mere leafless skele- 

 ton under the throttling grasp of its op- 

 pressor. So closely was it enveloped in the 

 meshes of the woody twiner, that I had to cut 

 my way through them with a hatchet in order 

 to take the measurements given above. 



While it prefers a rich, moist soil, as most 

 plants do when they have the choice, this ag- 

 gn^essive intruder can accommodate itself to 

 almost any conditions, trailing like an humble 

 creeper along the barren slopes of arid hill- 

 sides, rambling over wire fences along the 

 borders of dusty roads, from the cool slopes 

 on the plateau of Lookout Moimtain, to the 

 deepest ravines in the valley, and onward, 

 over the granite hills of the Piedmont region, 

 it has made itself at home. I could supple- 

 ment this case with some equally striking in- 

 stances of the rapid distribution of herbace- 

 ous plants, but it seems to me that the ex- 

 ample of a shrubby species which, in spite of 

 the fact that these are, in general, much less 

 efficient travelers than herbs, has been able to 

 naturalize itself, within the memory of people 

 now living, over an area extending from the 

 Gulf of Mexico to the estuary of the Hudson, 

 and for a thousand miles up the great Appa- 

 lachian Valley, may be taken as sufficient 

 evidence that other factors than time influ- 

 ence the distribution of species over a given 

 area. E. F. Andrews 



Roue, Ga. 



origin and development of the photo- 

 genic organs of photuris 

 pensylvanica 



There are at present three conflicting 

 views regarding the origin of the photogenic 

 organs in insects. One view is that they are 

 modified hypodermal cells, another that they 



are formed from both ectoderm and meso- 

 derm, and lastly, that they are mesodermal, 

 being derived from fat cells. Of these three 

 views that of the fat-cell origin has been the 

 most generally accepted. Moreover, recently 

 two important papers have appeared which 

 apparently definitely settle the question in 

 favor of the theory of fat cell origin. The 

 first of these was by Vogel ('12), who worked 

 on the embryology of Lanopyris noctiluca, 

 the other by Williams ('16), based on a study 

 of the embryology of our native species 

 Photuris pensylvanica. 



Unaware of Williams's work, I had under- 

 taken, at the suggestion of Dr. W. A. Riley, 

 a study of the embryonic development of the 

 photogenic organs of Photuris pensylvanica. 



During the summer of 1916, the eggs of this 

 species required, on an average, 26 days to 

 complete their development. 



These eggs cut in sagittal sections 3 

 microns thick, showed in the fourteen-day 

 embryos that the hypodermis on the ventro- 

 lateral portion of each side of the eighth ab- 

 dominal segment, in its anterior region, was 

 definitely thickened, due to proliferation and 

 enlargement of its cells. 



In the fifteen-day embryos the organ ap- 

 peared as a distinct nodule which projected 

 from the inner surface, though at this stage 

 there was no evidence of any separation from 

 the hypodermis. Further, it was found that 

 there was no evidence of any relation between 

 the fat cells and those of the nodule, in this, 

 or the fourteen-day embryos. 



In the sixteen- to seventeen-day embryos 

 the organ is completely separated from the 

 hypodermis, except at its two ends, where it 

 remains attached. From Vogel and Wil- 

 liams's descriptions of the earliest condition 

 of the light organ that they observed, one 

 would be led to believe that it was the study 

 of this stage of development, on which they 

 based their conclusions regarding its origin. 

 At this time the fat cells lie in rather close 

 proximity to those of the light organ and 

 somewhat resemble them. 



In embryos nineteen to twenty days old, 

 there occurs a differentiation of the cells of 



