Thrum

from all we’ve learned about pollination, you’d think a plant’s best bet for reproducing itself is to make it real easy on the pollinator. so shouldn’t most plants have so-called “perfect” flowers, that is, flowers bearing both male and female organs), with the anthers just as close to the ovary as they can get? it turns out that it’s often in a plant’s best interest to outcross with other plants within and among populations. constant self-pollination, and even pollen transfer among closely positioned (and thus often closely related plants) isn’t always the best for a plant’s offspring in the long term. with low genetic diversity in a population, it’s difficult to purge damaging gene mutations, and plants don’t

have a diverse set of genetic “tools” to enable them to evolve in response to changing environmental pressures. botanists call this phenomenon “inbreeding depression.” many inbred plants produce offspring that are less vigorous than outbred plants. an example is wild ginseng, panax quinquefolius, which is a rare, medicinal plant of new england woodlands. experiments have shown that inbred plants of this species are smaller and have lower survival than outbred plants; indeed, inbreeding depression may be one of the several causes for its rarity. we’ll return to this theme in week 4. violet species produce certain

flowers that self-pollinate before opening up (those so-called “closed” or cleistogamous flowers) and others that are open (chasmogamous) flowers which are fertile for pollinators. the features of these different types of flowers are actually useful for identifying particular violet species. so, many plants have floral structures that encourage outcrossing. that is, they separate male and female functions in time or in space. perfect flowers can require outcrossing if they stagger the development of their pollen and their eggs in time. that is, the anthers might be the first to develop, well before the stigma in the same flower becomes receptive. or vice versa. but if another flower in the same

population just happens to become fertile a bit earlier, pollen from another plant can fertilize it. some geraniums exhibit this temporal separation in maturing their flower parts. other plants with perfect flowers separate their stigmas and anthers in space, a phenomenon called “herkogamy.” primroses (species in the genus primula) have been especially well-studied, because they produce two kinds of flowers. so-called “pin” flowers, in which the stigma is elevated well above the anthers, attract pollinators who may be already transporting pollen from another plant and will drop it onto the obvious stigma. so-called “thrum” flowers have anthers that are much longer than the style, and most

pollinators will collect pollen from them and then fly to a new plant. producing both kinds of flowers at the same time means that pollinators carrying pollen from a thrum flower will promptly deposit it on a pin flower. clever, huh? other plants separate their reproductive organs more distantly in space, and produce male flowers, with only functional anthers and no stigma and female flowers with only a functional stigma. monoecious plants produce male and female flowers on the same plants. many trees are monoecious. an herbaceous example is our native stinging nettle (urtica dioica ssp. gracilis). i know…the species name “dioica” might make you think it’s “dioecious” (a term we’ll discuss next). interestingly, a

related non-native subspecies, urtica dioica ssp. dioica, is actually dioeceous. that’s how you can tell them apart. get out your hand lens, though: the flowers are tiny. we say a plant is dioecious when it produces exclusively male flowers on one plant and exclusively female flowers on another. that’s why, if you want holly berries on your holly plants (ilex opaca), you’ll need to plant at least two: one male, and one female. but what if sex isn’t always an option? or what if, even if it is, a plant really wants to hedge its bets (oh gosh, another terrible pun)? go vegetative! send a horizontal stem out into the world and grow a newplantlet off it.

many plants, such as this aptly named “running clubmoss” (lycopodium clavatum), spread via stolons. these are horizontal stems growing at or above the soil’s surface. others send out rhizomes (horizontal stems growing below the soil’s surface), and these can be some of our most stubborn species to remove, such as japanese knotweed (fallopia japonica). rhizomes are not just helpful for sending new daughter plants (called “ramets”) into the big, wide world. they also can transport nutrients garnered by the mother plant to the growing baby plant. cattail does this, and so does common reed (phragmites australis). incidentally, it was long thought that phragmites australis could pretty much only reproduce vegetatively, but recent

research by laura meyerson (whom you’ll hear from later in this course) and her colleagues has shown that it enjoys the best of both worlds - producing seeds and copious numbers of dense baby ramets. rhizomes also tend to store nutrients over the winter, sequestering them in their tissues, where they’ll be ready to feed the emerging plants in the spring. incidentally, some of those rhizomes are very nutritious for us, including ginger. nutrients are translocated from the photosynthetic leaves in late summer and fall, before they are lost when the deciduous leaves fall off. and this has implications for the timing of efforts to manage difficult plants such as phragmites. it’s often best to cut the stems just before the leaves have a chance to translocate

their nutrients, but after the maximum amount of nutrients have been gathered above ground. there are a few other ways to clone yourself, if you’re a plant. some flowering plants produce bulbils — tiny copies of the mother plant that may look like the flower buds but are not. these sometimes will even sprout rootlets while on the mother plant. eventually they will fall off and root elsewhere. this alpine bistort (bistorta vivipara) produces both flowers and bulbils. we learned earlier this week about the crazy reproductive cycle of ferns and their relatives, but i mostly focused on the spore-producing structures. but these

fascinating creatures also produce their own clonelets. cystopteris bulbifera (bulblet fragile-fern), shown here on the left, produces bulblets in addition to spores. and mountain firmoss (huperzia appressa) produces green copies of itself, on the upper half of the stem, called gemmae, while also bearing spores on the lower reaches. and, finally, land plants aren’t the only species that have evolved ways to reproduce themselves with or without sex. several aquatic plants, such as water-milfoil species (like myriophyllum heterophyllum pictured here) produce turions: detachable vegetative buds that can form new plantlets.

interestingly, some turions can go dormant for long periods — resting quietly in deep waters before being cued to begin new growth.sex, no sex, pollinators, clones... is it any wonder that plants have dominated the planet far longer than we have?

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