July Plant Fact

The obvious main function of leaves is photosynthesis.  Photosynthesis, in case those years of botany classes are too far in the past, is the conversion of Carbon Dioxide and Water into Starches and Oxygen in the presence of sunlight.  Come September, the focus of the article will be all about photosynthesis.  This ancient process both provides the building blocks for plants to grow and has modified the atmosphere such that we can exist!  However, some plants have developed specialized leaves that have functions beyond simply photosynthesis. 

Like flowers, the leaves of numerous plants also play a key role in reproduction.  A succulent is a plant which is typically native to drier environments and the leaves are typically thickened and possess water storing capabilities. This allows the plant to endure months with as little as 1/10 of an inch of rainfall. Conditions may not always be ideal for the production of seeds or for the seeds to germinate.  To counter these less than optimum conditions for seeds, succulent leaves or even a portion of a leaf will often root with ease once it makes contact with the ground.  One such plant that nearly everyone recognizes is Crassula ovata or Crassula arborescens (pictured below), the Jade Plant.  Native to South Africa, any leaf that drops to the ground has the potential to root!  Another, more close to home example isOpuntia humifusa, or Prickly Pear Cactus which is native from Montana east to Connecticut and south to Florida.  At the Gardens, a piece of an Opuntia leaf was somehow moved to another bed, where it promptly rooted!   

While the leaves of some plants root easily, the leaves of other plants actually produce young plantlets on the leaves!  This process is called viviparous reproduction.  Mother of thousands, Bryophyllum daigremontianum (also known as Kalanchoe daigremontiana) is a prime example, with upwards of 30 tiny plantlets being produced along the edge of a newly expanding leaf (pictured at bottom).   When the plantlets break off and fall to the ground, they quickly take root.  Although really cool to witness and they make great houseplants, in the greenhouse and in tropical outdoor plantings this species can be rather weedy. There are numerous other examples of viviparous plants, including water lilies and the house plant called Piggyback Plant or Tolmiea menziesii, both of which produce the baby plantlet in the center of the leaf!

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Crassula ovata
Bryophyllum daigremontianum

 

Other leaf modifications include the fascinating topic of carnivorous plants, with Pitcher Plants being one of the more interesting examples.  Pitcher plants normally grow in nutrient poor soils and the ‘pitcher’ is created through the infolding of a leaf to create a pitfall trap (Pictured on right).  The developmental pressure for the plant to produce such a leaf was due to poor soil nutrition.  These infolded leaves typically attract insects through the presence of red pigmentation (called anthocyanin) and/or nectaries.  The top rim of the ‘pitcher’ is moistened and made slippery by condensation or the secretion of nectar, causing the insect to slip and fall into the pitcher.  At the base of the ‘pitcher’ is a small quantity of a liquid in which the insect drowns.  The inside walls of the pitcher often feature downward pointing hairs, folds in the leaf, wax secreting cells or some other feature that prevents the insect from climbing up and out of the leaf.  In order for the plant to minimize the amount of rainwater that can enter the ‘leaf’, it has developed a ‘lid’ called the Operculum that is positioned over most of the opening.  Through bacterial action or the presence of enzymes in the basal liquid of the pitcher, the insect is slowly decomposed into amino acids, peptides, phosphates, ammonium and urea and from which the plant extracts the nutrients necessary for it to grow!  Pitcher plants are found both in the ‘Old World’ of SW Asia, Malaysia, and the Philippines east to Madagascar as well as in the ‘New World’ of North America.  The distinct difference between the two is where the pitcher is located.   In ‘Old World’ plants, it appears at the tip of an otherwise very normal looking leaf!  In the New World plants, the infolded leaf originates from an underground rhizome.  Regardless, it is a fascinating modification of a leaf to provide nutrition and to allow the survival of a group of plants in an otherwise inhospitable environment.

Beyond propagation and nutrition, other leaves help plants to climb for improved exposure to sunshine and thus, improved photosynthesis.  The petiole of the leaf is the ‘stem’ that connects the leaf to the stem proper.  It is also through this petiole that the sugars produced through photosynthesis can travel to the body of the plant and water and nutrients from the roots travel to the leaf.  Typically, the petioles main job, other than serving as a plumbing connection, is to support the leaf and make certain it is oriented to the sun in order to maximize efficiency of photosynthesis.  In various vines, the petioles actually twin around the supportive stem of a shrub while still orienting the leaf towards the sun!   A great example would be all the climbing forms of Clematis. 

Lastly, leaves can create entire environments!  In Bromeliads, the leaves are designed to hold water, such as in the Aechmea fasciata or Urn Plant (pictured above).  Principally native to the tropical regions of North and South America, it is estimated that 1 acre of a forest populated by bromeliads can store upwards of 5,000 gallons of water! In addition, it was found that a broad array of life thrives within the cup-like leaf bases of the bromeliads.  One study revealed that within 209 bromeliad plants, over 11,219 insects and animals including tree frogs and salamanders were identified, many of which are only found living in the environment created by a bromeliad leaf!  Whether producing Oxygen, creating new plants or providing a habitat for insects and animals, a leaf gives rise to life in a number of ways!

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Aechmea fasciata
Pitcher Plant