Morphogenetic Studies on the Distribution and Activities of Leaf Meristems in Ferns

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Abstract
In very young sporophytes of Dryopteris and Osmunda, the leaf primordia originate very close to the shoot apical cell and show early differentiation of an apical cell, rapid growth, and an early transition from distal to marginal growth. In successively older primordia of adult Dryopteris, a gradual elaboration in the size of the leaf apical cell takes place and the greatest size is attained before lateral pinnae begin to be formed. With the formation of lamina, the apical cell gradually decreases in size and is transformed into the marginal type of meri-stematic cell, when the leaf unrolls. In ferns with a homogeneous marginal meristem, which consists of a uniform layer of cells with an equal capacity for growth, a simple, entire leaf is formed, e.g. Phyllitis and Platycerium and where an initially homogeneous marginal meristem becomes heterogeneous, with a consequential differentiation of areas of unequal growth, a lobed or pinnate configuration, as in Blechnum and Lomnaria, or a compound leaf, as in Dryopteris, results. There are some indications of the inception of vascular elements being due to the activity of functioning meristems, the process being a basipetal one.