Volume 8 
Number 10 
The Plant World 
& j$taga?ine of popular ^Sotanp 
OCTOBER, 1905 
THE DEFENCES OF THE COCK-SPUR THORN. 
By R. G. Leavitt, Ph.D., 
The Ames Botanical Laboratory. 
The long, slender spurs of Crataegus Crus-galli are very sharp 
to the hand, but—if you wish to try their defensive quality best—• 
still sharper to the tongue. They are stems, springing from the 
axils of leaves. They bear no leaves except weak vestiges at 
an early period. These soon fall away leaving the tapering 
axes naked, smooth, and sharp. Trees of this species grow near 
our laboratory and while passing them recently I was struck by 
what appeared to be a marked downward tendency of the thorns. 
It seemed to me that the large majority were directed to points 
below the horizon. Yet, since their position is primarily deter¬ 
mined by that of the subtending leaves, and since these leaves are 
arranged spirally upon the branches, there was a probability that 
the thorns would on fuller examination be found to be spirally dis¬ 
tributed also and directed towards all quarters of the environment 
impartially. Between eye and logic only statistics could decide. 
Accordingly I counted the total number of thorns on several out¬ 
stretched branches, and then the number that pointed below the 
horizontal plane. This procedure demonstrated immediately the 
great preponderance of downward-pointing thorns. I discovered 
that many, in fact most, of the thorns were curved downward; so 
that even if originally directed upward at an angle of, say, 35 0 
above the horizon, they had turned from this elevation so as to 
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