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Species in the ExoticRainforest Collection.  
 

 

Regardless of what the folks at your local nursery or discount plant store told you the stalk that holds a leaf in place is not a "stem".   Look up the definition of the stem in a botanical dictionary and you will learn a stem produces roots and nodes.  Those nodes produce the stalk that supports the leaves and that support is technically a part of the leaf.  Take a look.  You won't find a root or a node growing on the stalk which supports a leaf so it cannot be a stem!  
Sound confusing? 
Although I doubt this article will change the use of the term "stem" as it has been adopted by house plant growers and in horticulture, regardless of the species any botanist will tell you the stalk that supports a leaf is correctly known as a petiole. Not a stem.
   
The stem is the plant's main support, base or central axis and its roots anchor the plant either to the ground, a tree or to a rock.  A stem may also be a rhizome as well as a tuber (often incorrectly called a corm or a bulb) and grow beneath the soil.  A stem may even spread as a repent rhizome creeping across the soil (repent indicates the rhizome runs roughly horizontally).  When you see a rhizome running across the surface of the ground that rhizome is the stem and may either grow underground or partially above the soil.

 

Plants such as Amorphophallus species have their stem (tuber) growing underground.  With only a few exceptions that tuber produces a single petiole that supports a single divided leaf.  Even though we correctly call the stem of that genus a tuber it is still just a stem that produces a single petiole which supports a single divided leaf.  Botanist Dr. Wilbert Hetterscheid is the world authority in the genus Amorphophallus and explains, "The tuber is indeed the strongly condensed underground stem consisting in most species of only one node being renewed every year with few exceptions existing."
 
I've attempted to explain this on many plant forums but someone will almost always come back and say the information I offer is either wrong or irrelevant.  Often they'll grab a definition from the internet without ever reading what the definition is attempting to explain or will change the facts to fit their own preconceived notion.  The use of a wrong name to describe one of a plant's most distinctive parts is virtually the same as calling a cat a dog or calling a boy a girl.  You wouldn't purposely call your child by the wrong name so why do many of us continue to call the petiole something that is not correct?

Even though I've researched this information for several years, just to be sure I am about to explaining this accurately I sent botanist Peter Boyce in Malaysia a quick note to ask "Is there ever a situation where the support for a leaf is correctly known as a "stem"?  Pete was in a hurry to catch a plane to Thailand but responded,  "The quick answer is never."  He then briefly continued while trying to get out the door saying the use of that term as a leaf support is "very poor use of the word stem."   Pete has described and published many tropical plant species in various genera of the family Araceae (aroids) to science and is one of the co-authors of the scientific text The Genera of Araceae published by the Royal Botanic Garden Kew in London. His co-authors are botanists Simon Mayo and Josef Bogner.

I sent the same question to my friend and mentor Dr. Tom Croat and he quickly responded,  "Steve, it is very simple. The stem has leaves which extend outward. These each have petioles and each petiole has a blade."   Please notice Dr. Croat explained each leaf has a petiole and the petiole is the stalk that supports the leaf blade. The problem appears to be the way individuals interpret the definition of a stem as it is often explained on the internet.

Some internet definitions clearly don't make a distinction between a stem and the petiole since the petiole is a part of the entire leaf structure.  That lack of a clearly understandable definition combined with the common incorrect usage of the world "stem" may have caused most plant collectors to use the term incorrectly. 

Again I asked Dr. Croat to explain about the parts of a leaf, "A leaf consists of a petiole and a blade. A petiole may also have a sheath and a geniculum. As a further twist the geniculum actually extends briefly onto the blade at times so is structurally a part of both the blade and the petiole even though we consider it to be technically a part of the petiole."  

When you read sites such as Wikipedia which includes statements such as "A stem is one of two main structural axes of a vascular plant" their statement is both true and scientifically correct but part of their further explanation can be confusing.  They include, "The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence (flowers), cones or other stems etc."  Their article is talking about the entire leaf unit, not just the leaf blade.  The "other stem" is known as a shoot as in the case of a rose plant.  The second main structural axis mentioned in the article on Wikipedia is the root system.  

A geniculum is not found on all plant species but is common on plants such as an Anthurium.  The geniculum allows the leaf blade to rotate to orient itself to the light just as your knee or elbow allows you to rotate your leg or arm.

A petiole can take on many shapes including to possess a round shape known as being terete or "U", "C" or "D" shaped when viewed as having been cut as a cross section.  The "U" and "C" shaped petiole are also known as being either sulcate or canaliculate  as in to possess a canal,  A petiole may even have very odd shapes such as being quadrangular or hexagonal and may even have patterns including lines which are known as being striate or grooves which are also known as being sulcate since the grooves form tiny canals in the petioles' axis.  A petiole may also be hard or somewhat spongy to the touch.   Botanists often use the shape and features of the petiole to determine the species of the plant.

The stem of a plant may be repent and run across the ground as a rhizome or or it may stand erect.  In many tropical species the stem often climbs a tree.  The stem is always the plant's base and produces buds, nodes and internodes.  The purpose is to collect and store water as well as nutrients which are absorbed by the roots. The water and nutrients are then distributed to the leaves through the plant's xylem via the petioles. The xylem transports water and soluble nutrients collected by the roots and stored in the stem  into the petiole and up to the leaf lamina.  The water is not "pushed" up the petiole but is instead "drawn" to the leaf lamina as a result of evaporation through the leaf blade.

 

Buds and roots grow from the nodes on the stem and the buds then produce the petioles. In many tropical plant species those roots never touch the soil since many grow suspended on the trunk or limb of a tree!  In climbing species the stem is also found clinging to the host tree and held in place by the clasping roots.

The use of the word "stem" to indicate the support for a leaf may have begun as a result of the beautiful rose. When we buy a dozen roses we find the flower at the end of a stem. In the case of the rose that support actually is a stem since it is a non-woody shoot which grows from the true stem which is likely climbing some support.  When we pick up a rose in a florist shop the leaves have almost always been removed so we forget those leaves also had a stalk support and that support was a petiole. The flower is a terminal flower on the end of a stem shoot which is very different from the support for any leaf so the rose flower is on the end of the stem but the leaves grow on their own petioles.

The stem is not the stalk that supports each leaf but is instead the main support of the plant and is composed of nodes and internodes producing roots, petioles and the sexual parts of the plant (flowers).  Just take a look at the stalk that supports any leaf.  You won't  find a node or internode growing from a petiole stalk. 

 

I'm sure the term "stem" will stay in common usage as the support of a leaf but at least you know the support of any leaf blade is not a "stem".   The support for a leaf is a petiole.  The support for the plant is a stem.

 

 

Although as non-technical as possible the information on this page is based largely on the scientific journals of botanist Dr. Thomas B. Croat Ph.D., P.A. Schulze Curator of Botany of the Missouri Botanical Garden in St. Louis, MO.  Dr. Croat (CROWat) has located an incredible number of newly discovered species and described them to science.  When you see the name "Croat" at the end of a plant's scientific name it was described and likely discovered by Dr. Croat.  Tom was honored in the summer of 2008 for having collected over 100,000 specimens in the wild.  Also, my thanks to botanist Peter Boyce in Malaysia as well as botanist Dr. Wilbert Hetterscheid in the Netherlands for their input and continued assistance.