The size distribution of sunflecks under a plant canopy can be represented by the length distribution of sunlit segments along a straight‐line transect drawn under the canopy. A mathematical connection is deduced between this sunlit‐length distribution and the probability, as a function of length, that a short line placed randomly under the canopy will be everywhere in sunlight. In turn, this probability distribution is deduced for randomly‐placed leaves, from which the above‐mentioned connection yields the desired sunlit‐length distribution. In subsequent papers this basic result will be used to predict the effect of penumbra — fuzzy shadow edges caused by the finite angular size of the sun — upon the light intensity distributions within or beneath canopies, as a function of leaf size, shape, height, density, and orientation.