How tall do annual plants grow?

Read any Flora, and you'll find each plant species' description starts with a statement about what kind of plant it is (tree, shrub, climber, or herb) and usually how tall it grows.  You might notice that a lot of the herbs (non-woody plants) are said to grow to about 80–100 cm tall.  Why is that, you might ask.

Arthur Healy (1917-2011), was a weed scientist at Botany Division of the Department of Scientific and Industrial Research, which was the Government's science research arm from 1926 to 1992.  Arthur's extensive herbarium collections of New Zealand weeds are the foundation for our knowledge of introduced plant biodiversity, but he wasn't just a collector, he experimented as well, and importantly he interpreted the significance of his findings for New Zealand.  One of his simple experiments was to plant the fluff that accumulated in his trouser cuffs on overseas trips, to see what seeds had lodged there.  Others were to see what grew out of packing straw and birds' nests.  These experiments demonstrated the need for more effective biosecurity at our borders and showed how weeds spread and got established.  They remind me very much of Charles Darwin's experiments.

Arthur Healy (DSIR)

But back to the weed collections.  Maybe it was a quirky interest, or maybe something that followed a plan, but Arthur collected the weeds that grew in odd urban sites like the cracks in footpaths and walls, or roof guttering.  And some of these were tiny, especially the annual herbs.

Annual herbs are ones that complete their life cycle within a year.  They get only one shot at flowering, so plants that die without managing to produce any seeds don't leave any offspring for the next generation.  Over the centuries, the annuals that succeeded were those that were genetically predisposed to flower and make seeds, however small and under-resourced they might have been.  Some of the weeds from footpath cracks and the shallow dusty soil of roadside gutters were much smaller than anything described in the Floras.  I remember Arthur's specimens of opium poppies just 1-2 cm tall, with tiny flowers a few millimetres across.  One had a seed pod that had room for just a single seed, all that starved plant could manage, but perhaps enough to pass its genes on for another generation.

Cirsium vulgare, called Scotch thistle in New Zealand.

When we were writing Flora of New Zealand Volume 4, Colin Webb and I noted Arthur Healy's collections of tiny footpath crack plants, and wondered about the other end of the range; what were the biggest plants we could find?  It became a common event to come in to work on a Monday and find a huge example of whatever genus one was working on lying on the office floor: Colin would leave giant brassicas or monster thistles that had to bend over at the ceiling of my office, and I'd leave 3m hemlocks or massive nettles covering the floor of his.

Hemlock, Conium maculatum.

When most botanists collect, they like to choose a sample that has flowers and fruits, and is healthy and representative.  Importantly, it also has to fit conveniently on a herbarium sheet, a standard sized piece of card, usually about 420 x 270 mm.  Tall herbs can be folded twice to be mounted on one sheet, but folded three or more times they clutter the sheet too much and make the specimen bulky.  Two folds cause the upper limit for a specimen to be rather less than three times as tall as the herbarium sheet; leaving some space for the label, that is about 80–100 cm tall.  So very tall plants are not well represented in herbaria, so long as there are conveniently-sized ones to collect.  Tiny plants are also not easy to collect.  Just a few on a sheet look silly, while collecting a few dozen and gluing them down in rows is a lot of work when a single larger plant can fill the space with ease.

Thus the two tails of the normal distribution of plant height might often get neglected for a herbarium collection, and this could be reflected in the plant sizes that are published in scientific descriptions.  By selecting the most aesthetically pleasing plants for the collections, are we biasing the information they hold?  (However, I checked a few plant sizes in Flora Europaea and Flora of New Zealand, and I think they're pretty good overall.  They do give the tall extremes at any rate, but maybe don't always include the smallest sizes.)