We present a model for unicellular algal growth as motivated by several experiments implicating the importance of calcium ions and ``loosening'' enzymes in morphogenesis. A growing cell at rest in a diffusive calcium solution is viewed as an elastic shell on short timescales. For a given turgor pressure, we calculate the stressed shapes of the wall elements whose elastic properties are determined by Young's modulus and the thickness of the wall. The local enzyme concentration then determines the rate at which the unstressed shape of a wall element relaxes toward its stressed shape. The local wall thickness is calculated from the calcium-mediated addition of material and thinning due to elongation. We use this model to calculate growth rates for small perturbations to a circular cell. We find an instability related to modulations of the wall thickness, leading to growth rates which peak at a finite wave number.