Several people commented on my previous post showing a picture of a new saguaro arm emerging from inside the saguaro (rather than a bud type growth). While wandering the saguaro forest this morning I looked around at new arms to see if my previous picture was an anomaly or not. From what I could see, it does appear that the new arms break out from under the parent column's cover. I took a close look at the saguaro above which had several young arms of different ages... Interesting.
Still signs of breaking out from inside the column.
Side view showing the tear out which seems to disappear as the arm grows larger.
And here's the tip of the start of an arm, currently a little larger than a golf ball.