Ceija Stojka’s small paintings at the Drawing Center swarm with such harrowing incident that viewers may not spot the mystery in the bottom right corner of many of her canvases. There, arcing over her signature, is a jagged black mark. Sometimes it forks like a lightning bolt. Only in a few of the paintings is the mark clearly discernible as the crooked branch of a tree. Born in Austria in 1933, the year Hitler took power in Germany, Stojka was a survivor of the Romani Holocaust, in which the Third Reich murdered as many as half a million people. In 1943 she and her family were sent to Auschwitz-Birkenau. The next year they were transferred to Ravensbrück, and then the next to Bergen-Belsen. In the despairing final months before the British liberated the camp in 1945, Stojka’s mother came upon a small green tree and fed its leaves and bark to her starving children, telling them, Stojka later wrote, “Here’s something good. It will make you strong.” She would remember the tree as her savior.…