Dissection of the marginal blastozone in Papaveraceae leaves

The poppy relatives (Papaveraceae), a family of 23 genera and around 240 species exhibit a broad diversity of leaf shapes. This diversity corresponds largely to different modes of dissection. Immediately after their initiation, leaf primordia are simple and undissected. Only during a limited growth period the primordium margins are competent to become fractioned into subunits that will form the leaflets, lobes and teeth of the mature leaf.

The tissue competent for this process (termed 'blastozone') often disappears already when the young leaf reaches a length of one or two millimeters. Nevertheless, distinct patterns of margin fractioning are seen during this brief organogenetic phase. These patterns appear to be regulated by other developmental processes such as differential dilation and histogenesis. I have documented the resulting patterns of leaf dissection in several species; some of them can be viewed here as animations (thanks to Katharina for the realization!).








By clicking on the leaf thumbnails, an animation is started on the left that shows the temporal-spatial pattern of dissection in this species. Starting with an entire outline (corresponding to the entire primordium), progressing dissection can be viewed until the final, real shape of the mature leaf is attained. Prospective sites of a fractionation are indicated by arrowheads. The drawings result from a mapping of microscopic observations onto the shape of a fully-grown leaf. At the time of these dissection events, the shape of leaf primordia is much less elaborated than seen in the animations.

Two phases of dissection can be distinguished: First, leaflets are formed along the leaf axis. Then, higher-order segmentation sets in that further subdivides the leaflets. Different species realize distinct patterns: In Roemeria refracta and Papaver pavoninum (subfamily Papaveroideae), leaflets emerge from the base to the tip (acropetally). Stylophorum lasiocarpum, Chelidonium majus, and Dicranostigma lactucoides are examples of basipetal leaves, in which leaflets are initiated from tip to base. This is also true for the lobes of Macleaya microcarpa (subfamily Chelidonioideae).

The wave of higher-order segmentation may also proceed acropetally along the leaf (and leaflets), as in Roemeria. In contrast, this process starts in the middle of the leaf in Papaver pavoninum and spreads to the tip as well as to the base (divergent pattern). The basipetal higher-order segmentation seen in Macleaya, Stylophorum, and Chelidonium, sets in soon after first-order segmentation, and is reiterated for several overlapping rounds; it is therefore less obvious at first sight. In these species, blastozone activity is maintained all over the periphery until it breaks off almost simultaneously. In Dicranostigma, higher-order segmentation is more delayed, and blastozone activity ceases not simultaneously, but basipetally.

Finally, you can view the mode of leaf dissection in Hunnemannia fumariaefolia (subfamily Eschscholzioideae). Architecture of the compound leaves in this group is polyternate. The pattern of dissection resembles that of Roemeria, but the two main lateral subunits basically repeat the dissection mode of the median subunit.