A potted plant is growing on a window sill in bright light The plant is rotated one-half turn each morning How would this turning affect the way the plant's stems grow?

Because if it didn't the leaves on the side not getting as much sunlight would not be able to photosynthesize as easily, where as if you turn it then each area of the plant will get as much access to the sunlight in order to carry out photosynthesis, providing you water it that is.
NOTE: Plants only ever have one stem - the long bit in the middle that holds it all together i.e is attached to the roots so is firmly planted in the ground, keeps the flower and leaves on too. The leaves are main parts in the ability to live for the plant as they contain chloroplasts in order for the plant to grow.