The Science of Botanical Art

Stems 

By Dick Rauh  

Originally Appeared in The Botanical Artist, Number 34

 

Last time we started our discussion of vegetative morphology, looking primarily at roots. Now I want to move up on the plant and look at what is happening above the ground line, focusing this time on the stem and next on leaves. 

The whole plant is considered the habit, and whether or not we include roots most written descriptions of plants start out with a description of the ‘habit’. Habit breaks down to two basic forms; herbaceous or woody and that needs some further clarification.

What makes a plant ‘woody’ is a phenomenon called secondary growth. I am not sure if I stressed this before, but in shoots as in roots the primary unit of cell division in a growing plant is a group of small, undifferentiated, unvacuolated cells named meristems that exist in the tip (or apex) of the shoot. These shoot apical meristems are the only actively dividing cells in the stem of a growing plant, and they cast off daughter cells now called initials, in a downward direction. (These cells are the plant world equivalent of the stem cells of the animal kingdom that we have been hearing about so much in the news). Now if I may borrow a locution from my realtor friends, what type of tissue these initials develop into depends upon location, location, location. 

This is a hard one to swallow, but as artists, try to visualize this; a group of cells in the tip of a stem being pushed up by the growing initials they have just created. Not only are these initials enlarging and elongating until they reach their final size, but they are differentiating, taking on characteristic of a particular tissue, primarily dependent on the position they end up. The cells that are cast off to the outside of the shoot become the epidermal tissue. The cells that are cast off to the center of the shoot become vascular tissue, and the cells cast off in between become the cortical tissue or cortex. Once these cells reach their mature growth they stop growing. Period. It’s almost like the building of a brick wall if you can imagine this in an organic context. This accounts for the vertical growth in most plants. 

I don’t want to confuse you, but meristematic growth occurs at the tips of each lateral branch, and at each leaf bud too, but in each case it functions in a similar fashion (I would feel remiss if I didn’t let you know of plants where this meristematic cell division occurs in different places than the apex, {often in the base of a leaf-as in grasses} but now that that caveat is mentioned, let’s forget it). 

For herbaceous plants this type of shoot apical meristem is the primary growth mechanism. Woody plants do indeed employ this primary method, but another ‘secondary’ growth distinguishes them. This is a lateral enlargement or girth (much more desirable in plants than around our waists), and a layer of cells that encircles the interior of the stem achieves it. This is the cambium; a single row of actively dividing cells (meristems) throwing off in this case, xylem cells to the center, and phloem cells to the outside. Outside of this are living and dead tissue we know as bark. This said, it is easy to name the types of woody habit; trees, shrubs and the woody vines known as lianas. 

Herbaceous growth on the other hand, is often characterized by the temporal growth habit itself. Does the above ground plant die back at the end of the growing season to the roots, only to revive with the next growing season, for an indeterminate number of seasons? A perennial. Does the growth habit of the plant need a two-season cycle to complete, sending up first a vegetative plant then one with reproductive organs the next season? A biennial. Does the plant complete its entire life cycle in on growing season? An annual. (By far the most successful of all plant habits). Aquatic plants, climbing plants and plants with basal rosettes are additional variations of the herbaceous habit, and there are more. If at some point you are asked to illustrate the ‘habit’ of a plant, hopefully you will know what your client is talking about. 

The functions of a stem are multiple. Mainly it is responsible for the support of the plant, and we need to look closely to note how rigid, or flexible it is, perhaps ribbed or winged. Most stems are terete (rounded in cross-section) because they contain the vascular column which is its second function- that of transport. Some families are characterized by stems that are square (the mints) or triangular (the sedges) but within these still remain the rounded vascular column.

Stems, because they have nodes and lateral buds are also propagative units. Under the proper conditions planted cuttings of stems have the potential for producing whole new plants, a function of which roots or flowers are incapable. As in roots, however, thickened stem tissue has been adapted for storage. Corms, the reproductive unit of Gladiolas or Crocus, are flattened belowground stems. Rhizomes are belowground horizontal fleshy stems, and Iris for example uses this storage organ, as do many ferns. 

Sometimes the horizontal stem is above ground, complete with nodes and buds, and can be fleshy, as in the stolons of rabbit’s foot ferns, or threadlike as in the runners of strawberries. A fat underground stem is a true tuber, as in white potato and sweet potato. The ‘eyes’ are the nodes and lateral buds, indicating this is stem tissue, and capable of propagation. 

It might be fun to look closely at off-season twigs (see figure), and to see how much taxonomic as well as morphological information can be gained. It might even inspire you to do some drawing or painting when there isn’t much action outdoors otherwise. A book by Earl Core and Nelle Ammons, Woody Plants in Winter, deals exclusively with this an identification technique. Check out: the delicacy or coarseness of the twig; the look of the terminal bud or buds and their scales; the position of the lateral buds; are they alternate or opposite; the shape of the leaf scar and the pattern of the bundle scars (you will need a loupe for this). Note the prominence or subtlety of the lenticels, these little pockmarks on the surface of a young stem that are its breathing apparatus. And try to find an irregular series of lines that circle the twig. These are the terminal bud scale scars and mark the start of a year’s growth. 

Enough for stems for now, now that we have a few more names to add to our list. We know a few already. If it is the stem of a solitary flower, it’s a peduncle. If it’s the stalk of a single flower in an inflorescence it’s a pedicel. If it’s the stem of a leaf it’s a petiole (more about this next week). A horizontal underground fleshy stem is a rhizome, above ground this is a stolon, and if it’s skinny it’s a runner. The bark-covered stem of a woody tree is a trunk, but the primary supporting axis of most plants is a stem.