The berries in my yard this summer were incredible. Our blueberry crop was the best ever, and the blackberries and raspberries were also large, juicy and plentiful. Picking fresh berries brings back memories from my childhood and continues to provide joy as well as delicious treats.
However, there is one exception to all the fun: blackberry thorns. Or more accurately — prickles. Whatever they are called, they add to the price I pay for berries, not in money but in blood and pain!
Botanically there are three main types of sharp appendages that plants produce to protect
themselves. Prickles are an outgrowth of the stems and leaves and can be found on blackberry and raspberry plants. Prickles also grow on rose canes, tips of holly leaves, stems of the devil’s walking stick and thistles. They are generally smaller than some of the other spikey defense mechanisms produced by plants. Because they do not grow from the wood of the plant, they can be peeled off with the epidermis.
By contrast, true thorns are modified stems which terminate with a point. They grow from a node where a branch should be and are found directly above a leaf or leaf scar near a lateral bud. Thorns may be singular or branched, just like any other stem. Examples of plants with this type of defensive protrusion include honeylocust, hawthorn and some crabapples. Because thorns are woody and stem-like, they tend to be stiffer and more robust than prickles.
The third type of sharp plant material is spines, which are modified leaves. They are found where a leaf should be, just below a lateral branch. Look for spines on black locust which has paired spines at the base of a leaf. Gooseberries have spines, as do cactus plants.
Sharp parts aren’t the only way that plants attempt to protect themselves from defoliation. Some defenses are actually soft. Fuzzy leaves such as those found on mullein or lambs’ ear can be a deterrent to insects and mammals. Waxy leaves not only help a plant retain moisture, but can also be less appealing to grazers.
Rough and thick bark makes it more difficult for insects or browsers to penetrate a tree and access the nutritious cambium, or growth layer beneath.
Some plants such as milkweed contain a bad tasting and toxic sap. Contact with the milky substance which gives the plant its name can cause eye or skin irritation. The sap makes leaves taste bitter, so mammals and insects don’t eat more than a bite or two. Monarch butterfly caterpillars are a notable exception, depending entirely on milkweed for their larval diet. They have adapted to being able to consume the sap, and in fact the toxic glycoside remains in their system to make them unpalatable as well. This is a great example of passed on protection!
Poison ivy also has an infamous sap which offers the plant protection. The irritant usually
causes a skin rash for anyone unfortunate enough to brush against it. Imagine if you were an herbivore who ate some. That gives a new perspective on the idea of a scratchy throat! Learning to identify this plant is one of the most important lessons of woodland explorers in order to avoid the discomfort it can cause.
Other sap may be deceiving in its impact. The wintergreen flavored sap of black and yellow birch trees seems appealing. Yet for browsers such as deer and moose, consuming large quantities of these branches can lead to digestive discomfort.
Research has shown that when some plants are heavily browsed, they give off chemicals in the form of volatile organic compounds into the air, or other substances released via their roots that can be detected as a warning by neighboring plants. These nearby plants may increase their concentrations of toxic substances to ward off the approaching consumer.
Defense mechanisms such as these serve to reduce consumption but generally don’t entirely prevent animals from tolerating the pain or discomfort to benefit from the nutrition or taste of the plant. They likely decrease the amount of fruit or foliage that is consumed, enabling the plant to continue to grow and reproduce, resulting in both the plant and future consumers (like some determined berry pickers) to be successful.