Self-adjusting snowdrops

Galanthus 'S. Arnott' bulbs with elongated stems to adjust their position in the soil.

 When I was in Washington D.C. last month, I was asked to comment on how bulbs adjust their depth in the soil. It is quite well known that some 'bulbs', including Scilla and Crocus, can form contractile roots that can pull the bulb deeper, and in tulips and Erythronium 'droppers' are formed, with the new bulb forming on the end of  a shorter or longer stem or stolon. The phenomenon of bulbs pulling themselves upwards seems to be less well-documented, and it was this that stimulated the discussion, as some Lycoris bulbs had been observed to be doing so. Back home, with  alot of snowdrop bulbs to prepare for potting and replanting I found, as expected, some bulbs showing evidence of having done just this.

As a snowdrop (in this case) clump develops, a mass of superimposed bulbs is formed, some of which, at the lower edge of the clump, or packed in the middle, may need to 'escape' to a more suitable depth. Unfortunately the clumps were already broken up by the time I looked for examples, so I can't illustrate this. Also, sometimes bulbs become buried too deep by mechanical means, and must also readjust their position in the soil.

On the left is the new bulb formed in the current season, large enough to flower in this case. The new basal plate is visible as a yellowish patch at its base, above the extended stem (now withering) that started at a basal plate at the right of the image. The stem is surrounded by the withered bulb scales from the previous season's growth.

What seems to happen is this: in the previous season (year 1) the bulb must somehow 'recognise' that it is buried deeply, and either while still in growth or shortly after it becomes 'dormant', extends the normal short and compact stem (basal plate) to which the bulb scales and roots are attached into an elongated structure potentially several centimetres long. This can only happen at these stages, because the growth in the current season (year 2), starts at the end of the extended stem. It's the only place a main snowdrop shoot can start from. As expected, a bulb develops from the shoot and is thus attached to a long, now redundant and soon withering stem. The old bulb scales, from year 1 (and possibly earlier) wither away in year 2 and form a sheath around the elongated stem, not around the new bulb as they would if it had formed in its normal position (further proof that the stem elongates at an early stage of shoot development). The new (year 2) bulb lacks these protective scales and thus always appears white (and is prone to desiccation). The result is the very curious morphology shown here, in which a long narrow stem bears a normal bulb at its tip, a few centimetres higher in the soil than in the previous season- the reverse of the 'droppers' mentioned above, but essentially by the same means.

No doubt this process is well-known to plant morphologists, but it has been fun to dissect some bulbs and deduce the situation for myself. Now I must attempt some controlled experiments to study the process as it happens.