void setup() {
  size(600, 600);
  background(0);
  stroke(255);
  drawTree(300, 550, -90, 9);
}

void drawTree(float x1, float y1, float angle, int depth) {
  float forkAngle = 20;
  float baseLen = 10.0;
  if (depth > 0) {
    float x2 = x1 + cos(radians(angle)) * depth * baseLen;
    float y2 = y1 + sin(radians(angle)) * depth * baseLen;
    line(x1, y1, x2, y2);
    drawTree(x2, y2, angle - forkAngle, depth - 1);
    drawTree(x2, y2, angle + forkAngle, depth - 1);
  }
}