If you walk along the 19th Street side of the main branch of the Philadelphia Free Library on the Benjamin Franklin Parkway, you’ll see a row of plants that, though composed of three different species, all have a certain set of similarities. And if you walk along the 20th Street side of the library, you’ll see the same thing – three different kinds of plants, one somewhat tall, a tree about 15 or 20 feet high or a bit more, and another, a bit smaller, in the 10 to 12 foot range, and then another, a smaller bush, about waist high.
One of them, the tallest, is a hawthorn, a Crataegus, that is most likely the Washington cultivar (given that its leaves are pretty big); an other, the smaller sized one, is Pyracantha, which sometimes goes by the name “firethorn”, but in my experience more often just goes by “Pyracantha”; the third kind of plant there is the toothache tree, or prickly ash. There are a number of names and three different sizes there, but these plants share some similarities.
All of them, as you will see if you go by there about now, have bright red berries – attractive to birds, serving for dispersal, they also make for a cheery red that draws the eye to this row of plants, especially now as leaves are falling. They also all have sharp structures jutting out from them – two of them have thorns, one of them has spines, and all of them are armed.
What are spines, and what are thorns, and how do they differ? To explain this, we need to back up a little bit, to explain how plants work, how they develop, how they grow.
Plants are modular – they grow in sections (modules) along the stem, and each section contains a node and an internode. The node is where the leaf and branches or buds come out, and the internode, as you might guess, is the part of the stem that is between the nodes. At the nodes is where we have leaves coming out, and also branches or buds. If you see a leaf, right next to it there will be an associated branch or bud, or a scar marking where one had been. And if you see a branch or a bud, right next to it there will be an associated leaf or a scar marking where one had been. And their arrangement is standard – the branch or bud or their remnant scars will be distal to the leaf (distal means toward the tip of the main stem), and the leaf or its remnant scar will be proximal to the branch or bud (proximal means away from the tip of the main stem). If you look at a plant, you will see this pattern.
There’s an additional set of structures that are important to our story here – stipules. Stipules are leaf like structures at the base of a petiole (a petiole is the stem of a leaf), and while we don’t know quite what they do for the plant, they frequently help us (botanists, that is) identify plants, since their presence or absence, or shape or size, can be diagnostic for certain species.
These various organs can be and often are modified, evolutionarily, into different structures – a branch might get sharp at the end, a leaf might gain points, stipules might turn into a defensive arm… and this is what we infer has happened in the ancestors of the plants that now grow along the library.
Hawthorns have thorns, as you might guess from the name – thorns are modified branches, and so if you look at the base of a pointy thing on a hawthorn, and look at the side that faces away from the tip of the branch (the one upon which the thorn is inserted), you’ll see either a leaf or a leaf scar. Or, for some thorns on a hawthorn, they will be at the very end of the branch – that is, the tip of the branch itself is modified into a thorn.
Pyracantha have thorns, too, which you can see evidence for yourself if you look at their bases, to see the remnant leaf scars, or leaves even, perhaps, if they persist. Or you will see that the thorns are at the tips of branches, for some of them.
The toothache tree, distinct from the species mentioned above, has spines – stipular spines, actually.
You might think at first, that the toothache tree has prickles. Prickles are outgrowths of the epidermis that are, as are thorns and spines, sharp at the end. You can find prickles on roses, or blackberries, or raspberries – and you can differentiate them morphologically from thorns or spines because they (prickles, that is) are found throughout the internodes, as compared to thorns, which occupy the geography of the branch, from which they are evolved, or spines, which are where a leaf would be – both thorns and spines are at the nodes, prickles are in the internodes.
If you look at a toothache tree that’s aged a bit, it seems that the sharp points arise throughout the internodes, which would make you think that they’re prickles. However, if you look at younger branches, and look at the bases of the leaves there, and you select a series of them from younger to older, you’ll see that these spines are developed from the stipules, at the bases of the leaves. They’re stipular spines, as you can prove to yourself by looking.
And so, here by the library, we have this trinity, repeatedly growing in lines up 20th Street and down 19th Street – three different species representing two different families (Pyracantha and hawthorn are in the Rosaceae, toothache tree is in the Rutaceae), all with bright red berries and sharp points (which develop and have evolved distinctly across these family’s evolutionary lineages).
The bright red berries are attractive, but the sharpness protects them – an interesting metaphor for a landscape that encircles a library, I think: the fruit of the tree of knowledge is attractive, but there is a cost, it is sharply protected by the branches where you find it – and whoever gets in there to take it in, to eat the fruit, is then in its service to disperse it, to thereby perpetuate it.
I have no idea if this metaphor was intentional on the part of the landscape designers who originally decided on these plantings decades ago, and it may well be that they just wanted plants with bright red berries in the fall and into the winter, and that they wanted a variety of heights to provide texture to this urban landscape, and that what was available from the nursery all had thorns or spines.
But nevertheless, they provide a keen starting point for dispersing an explanation of the differences among thorns, spines, and prickles.