Friday, April 19, 2019

Theme and Variation - the Amaryllidaceae

The primary types of cultivated amaryllis are in the genus Hippeastrum.  
Their flowers are mostly shades and mixes of red, pink, and white. 
This is  one of my favorite cultivars, "Eye-catcher."
This spring, while I was waiting eagerly for the amaryllis plants in my yard to bloom, I started reflecting on
the marvelous family to which they belong, and how nicely they represent a fascinating aspect of plant evolution.

The Amaryllis family is known and beloved worldwide, even by people unfamiliar with its technical name or taxonomy, for it provides us with a variety of unique spring-flowering bulbs and perennials, from daffodils to subtropical amaryllis and tropical Crinums.

As presently defined, Amaryllis (technically the genus Hippeastrum), daffodils (Narcissus) and Crinum all belong to the subfamily Amarylliodeae. Onion, garlic, etc.are also members of the family, constituting the subfamily Allioideae.   Finally, the blue-flowered "Lily-of-the-Nile" (Agapanthus), from southern Africa, is technically in it's own subfamily, Agapanthoideae.  Altogether, there are some 1600 species in 75 genera, found naturally on every unfrozen continent.
Daffodils are specialized members of  the genus Narcissus, in which the umbel has been reduced to a single flower.

The subfamily Agapanthoideae consists of the single genus Agapanthus from
southern Africa. Flowers are blue to white.

The onion subfamily,  Allioideae, contains numerous aromatic and edible species.
The characteristic pungent fragrances are based on allyl sulfides,
which in nature  act as deterrents to insect pests.

The true bulbs of onions and amaryllis are made up of
the swollen bases of recent leaves that encircle the
central stem.  The outermost layers, representing
 older leaf bases, become dried and paper-like,
 which protects the fresh inner layers from drying out.
 This gives rise to the designation "tunicate bulbs,"
differing from the scale-bulbs of the true lilies.
Photo by Amada44, CC BY-SA 3.0.

So what defines this family? What is the common theme upon which the 1600 species are variants? The vast majority of the species in this family are geophytes, plants that survive adverse seasons underground. Most species form bulbs, but some, like Agapanthus and certain members of the Allioideae, employ underground rhizomes instead.  The leaves are strap-shaped (sometimes tubular and hollow in the onions) and extend themselves upward from the bulb by basal intercalary meristems (see "How the grass leaf got its stripes").  This is the most common form of leaf in the monocots, and it varies little in this family.

True, or tunicate bulbs (see illustration to the left), differing from the scale-bulbs of the true lilies, do seem to be a unique invention of this family,  though some members of the Lily family, such as tulips, have evolved a similar type of bulb independently.

Flowers in the Amaryllidaceae  undergo preliminary 
development below ground, between leaves or within the 
bulb and are protected by a closed sheath.  The enclosed 
bud is then pushed upward by the intercalary growth of  
the stalk.

But it is how the flowers emerge from the bulbs that is the most iconic, revolutionary, and consistent theme of the family.  Flowers form below ground, tightly enveloped in a protective sheath.   Below each inflorescence bud, a stalk (the peduncle) develops and lengthens through basal intercalary growth (i.e. new tissues are produced at the base of the stalk, pushing older tissues and the inflorescence bud upwards).  After rising to optimum height for pollination and eventual seed dispersal, the sheath splits open to reveal a simple umbel, i.e. one to many flowers arising from a single point at the tip of the stalk, roughly forming the shape of an umbrella or sometimes an entire sphere.

This proved to be a remarkably effective way to protect and elevate the flowers, for after it evolved in the common ancestors of the family, descendant species spread worldwide, adapting to different climates, soils, and pollinators. Such a spreading diversification is called an adaptive radiation. Note that the special structure and growth form of the inflorescence remained essentially unchanged throughout the family, while details of flower structure and color, fruit type, and physiological adaptations diversified.

Yellow flowers are uncommon in the Amaryllidaceae, but found here in
Lycoris aureus.  Photo by Tomago Moffle, CC BY-SA 3.0.
 The importance of this discussion is not simply to say how wonderful and unique the Amaryllidaceae is, but to stimulate us, particularly those of us who are teachers, to look for similar patterns of breakthrough adaptations followed by adaptive radiation throughout the plant kingdom.

Almost any genus, and sometimes a whole family can be seen to be based on some "great idea," i.e. some new structure, growth pattern, flower type, etc., that gave the ancestral species an advantage and allowed its descendants to diversify into great numbers.  Two simple examples are the genus Aquilegia (columbines) with its nectar spurs arising from each of the five petals, and the genus Euphorbia, with its highly compact flowering units called cyathia.

How many examples can you find? Can you explain the adaptive value of the distinctive features?
Each yellowish, red-tipped structure in this Poinsettia
(genus Euphorbia) is a cyathium, a cupule containing several
tiny flowers.

The highly distinctive flowers of Aquilegia feature a nectar
spur projecting backwards from each petal.

The giant crinum, C. asiaticum, from southern China, is an tropical evergreen
plant that develops a pseudostem, similar to that of the banana, made of the
tubular bases of the leaves.