Appendix III - Stems and Twigs


Contents:

Part 1: The Difference Between a Twig and a Stem

Part 2: Buds

Part 3: Leaf Scars

Part 4: Lenticels

Part 5: Other Noteworthy Characteristics

Part 6: Pith

Introduction


Twigs, stems and the buds on them are often ignored in woody plant identification because distinguishing characters can be relatively subtle and using them requires some practice. However, in addition to leaves, stems and twigs can be very useful for species identification. The appearance of leaves can vary a lot depending on genetic factors, environmental conditions and the stress level of the plant. For example, leaves in the sun can look very different from shade-leaves as they have to cope with harsh, direct sunlight. Deep lobes on the laves, hairiness or a waxy covering are often found on sun-leaves and help protect photosynthetic cells from overheating in excessive sunlight. Stems and twigs are important for identification in temperate climates, because the leaves of many trees and shrubs in regions such as the Mid-Atlantic are unavailable during the winter months.

In the following illustrations, we present basic terminology of twigs and stems and how the different components may be useful in pant identification.

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Part 1: The difference between a stem and a twig


In the interactive key, we will refer to those parts of the tree with foliage as the stems and the term twig is reserved for the current season’s growth. Note that other sources may refer to the current year’s growth as a stem and not as twig. Usually, the stems used for keying are the second and third year’s growth. Differences in color and/or pubescence (hair) between twigs and stems and between species can be useful for plant identification.

So lets learn to distinguish between the current year’s growth (twig) and last year’s growth (stem). We do this by finding the bud scale scars. Scales form protective coverings over immature leaves and sensitive growing tips (buds) at the outer most points of branches. Scars are left behind when the scales from last year's terminal bud dropped off as new leaves expanded in the spring and the twig grew throughout the season (see pictures 1 and 2).

Picture 1: The cluster of lines encircling the stem are the terminal bud scale scars. The current year’s growth (i.e. the twig) is the portion between the scars and the very tip of the stem. The growth below the bud scale scar is previous year’s growth (i.e. stem).
Picture 2: Terminal bud scale scar and opposite leaves. Note also the distinctive color difference between the twig and the second year’s stem.

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Part 2: Buds


Buds contain the undeveloped shoot, leaf, and/or flower. The arrangement of the buds on the twig and the scales on the bud, as well as color, pubescence, size, and shape of the bud can be critical for identification. A bud at the very end of a twig is a terminal bud (Picture 3). Buds along the sides of a twig are called lateral or axillary buds (Picture 3). Terminal buds release hormones that inhibit the growth of lateral buds resulting in new growth at the end of the twigs with maximum exposure to sunlight for photosynthesis.

Some woody plants lack a terminal bud, but have two or more lateral buds at or near the tips of twigs. The presence or absence of a terminal bud can eliminate potential choices for identifying a plant. There are some exceptions of course, and in pseudo-terminal buds the buds only appear to be terminal but are not really. For example, in some plants the twig dies back and falls off close to the nearest lateral bud that then resembles a true terminal bud (Picture 5). Sometimes a short stub remains and gives a clue that you are not dealing with a true terminal bud. A pseudo-terminal bud will also have a leaf scar below it while a true terminal bud will not.

The arrangement of lateral buds will also provide identification clues for you. Certain species have multiple buds side by side at one node (called collateral buds) and these are one shoot bud surrounded by two flower buds. Buds that are stacked on top of each other are superposed.

The buds of most Mid-Atlantic woody plants are covered in scales that protect the embryonic tissue from freezing (Picture 6) and produce the bud scale scars covered in the previous section. The number of scales, their arrangement, color, presence or absence of hairs or sticky substances are often distinctive. A few families and genera in this region (e.g., members of the family Annonaceae and viburnums) have buds that lack scales, which are termed naked buds (Picture 5). These buds can be very hairy ( pubescent).

Picture 3: A twig with a terminal bud at the end and lateral or axillary buds (in the leaf axils) on the sides.
Picture 4: A twig without a terminal bud. There are only two lateral buds and the stubby remnant of the stalk that held the flowers.


Picture 5: The naked bud of a viburnum without bud scales, but pubescence for protection.
Picture 6: Buds covered with protective bud scales.




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Part 3: Leaf Scars


When leaves fall, scars remain that have characteristic shapes and sizes. Most often, leaf scars are directly below a bud, but occasionally they can be placed to the side or above the bud.

Within the leaf scar, you may see a number of dots and this is where vascular bundles passed from the twig into the leaf (Picture 7). Vascular bundles are the conduits where water, nutrients, and the products of photosynthesis travel to and from the leaf. The pattern and number of bundle scars left by the vascular channels as the leaf dropped off are important identifying characteristics.

Picture 7: A buckeye twig showing a sizable leaf scar and 7 vascular bundle scars.
Picture 8: A uniquely shaped hickory leaf scar with 3 bundle scars that resemble a monkey face. Note the lateral bud to the top of the leaf scar.



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Part 4: Lenticels


Lenticels are corky pores on twigs or bark that allow gas exchange between plant tissues and the atmosphere. These pores usually look like elliptical bumps on woody plant parts and can be characteristic for some genera. Cherry trees, for example, have very distinctive lines of lenticels on their bark (picture 9).

Picture 9: The bark of a Yoshino cherry showing distinctive horizontal lines of lenticels.
Picture 10: Conspicuous lenticels on a twig.



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Part 5: Other Noteworthy Characteristics


There are additional useful characteristics on stems and twigs, such as thorns, spines, or prickles that arm plants against browsing animals such as deer.

Thorns are modified branches and are found on hawthorn for example (Picture 11). Spines are modified leaves or stipules (Picture 12). Prickles are outgrowths of the epidermis, or outer most cells layers (essentially the 'skin' of a plant) (Picture 13).

There are a few plants with corky ridges on their stems, and these are often called wings (Picture 14). Sweetgum trees occasionally have winged twigs, but likely the most familiar winged plant in our region is the compact winged euonymus or burning bush.

Other plants have stems where the bark is peeling or flaking off (exfoliating (Picture 15).

Picture 11: Thorns are modified stems.
Picture 12: Spines are modified leaves or stipules.

Picture 13: Prickles are outgrowths of the epidermis.

Picture 14: Corky wings on a burning bush.
Picture 15: Exfoliating bark.



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Part 6: Pith


Internal features of stems and twigs can be just as useful for identification purposes as are the easily visible parts. Splitting a stem or twig with a knife or pruners can reveal colors and patterns that are unmistakable. The pith is the spongy central portion of a twig and can be seen easily in cross section or if the twig is cut lengthwise and can be brown, green, orange, white or yellow. Additionally, the pith can be continuous (i.e., if it is solid throughout its length) or it may have disappeared and left an empty space, in which case it the twig or stem is hollow. If there are partitions of tissue with different textures or color at regular intervals with continuous parts, the pith is diaphragmed. When there are intervals of tissue with hollow chambers between the partitions, the pith is chambered.

Picture 16: A chambered pith.