And here's a thorny plant up close. They're distinguishable by their purplish canes (stems) and those bright green leaves.
As I was working to eradicate these thorny plants from the garden, and later as I worked on this little patch of land near the deer trail, I began learn how these annoying buggers operate. As you can see above, they can spread out in multiple directions from their launching point. The shoots first grow straight up, without thorns (as I showed in a photo yesterday), but eventually the cane thickens and develops thorns, as it bends back down toward the ground. When a thorny plant cane hits the ground, it begins to lay down roots. These roots appear to grow pretty quickly, as even a thin cane-end can be attached to a well-developed root system, as shown below. This new root can then become a new launching off point for more canes, as these aggressive plants march ever onward in their efforts for the conquest of my yard.
These thorny plants are stubborn, as they will grow pretty much anywhere. See the purple shoot coming straight out of the short tree trunk in the photo below?
I sent one of the above pictures to Craig Cramer, who works for Cornell's Department of Horticulture, and who runs an excellent garden blog, Ellis Hollow. Now, I was expecting to hear that this was some evil, invasive plant from overseas with some stark name like Purple Death. His reply was not at all what I was expecting...he suggested that these evil, thorny plants were wild raspberries. I did some research on the web, and it turns out that these are indeed most likely wild black raspberry plants. Of course, I had missed the flowering and berry stages because they happen in late spring to early summer, and I didn't move here until August.