There has to remain some small branches and leaves near the bottom or the plants won't grow back. Ultimately though plant regeneration is possible, MJ by nature is an annual. Growing from seed to maturity in a single summer season, dropping seed and then dying off. Once bloomed to maturity and harvested the plants lose much of their life energy and most importantly, potency. Regenerated growth is slower or less vigorous and potent, leaving a much less desirable harvest the second time around. This is why most growers clone from a mother plant that's kept in a perpetual veg stage.