Stele

As a child, I would eat around the center core of my carrots, leaving the darker, sweeter core for last. I didn’t know it then, but that inner core is called the stele.

Carrot steles are far more visible in purple carrots (Kate Russell)

Vascular plants have both root and stem steles, but they didn't start out that way. Primitive steles were nothing more than a strand of xylem, surrounded by phloem. [Remember, water and minerals ‘rise up the xylem’ from the roots, and manufactures sugars ‘flow down the phloem’ from the leaves. In case you forgot.]

More modern steles may consist of vascular tissue, pith, and pericycle. Pith is the spongy material seen in the center of stems, and the pericycle is a thin layer of tissue between the xylem and the endodermis. There are two major types of stele: protostele and siphonostele.

Types of protostele (EncycloPetey) Public Domain

Protostele

​Protostele describes the more primitive stele, which consists of a strand of xylem, surrounded by phloem.  Protosteles may or may not have an endodermis that controls the flow of water. There are three different types of protostele:

• haplostele - a single core xylem, surrounded by phloem
• actinostele - a lobed or fluted haplostele, commonly found in mosses
• plectostele - plates of xylem run in parallel

Siphonostele

Siphonosteles are a little more complex than protosteles. Siphonosteles may have gaps in their vascular tissue in places where leaves are born. These spaces are called leaf gaps. You can think of these leaf gaps as sections cut from a hula hoop and pulled a little apart, making room for leaf tissue to grow through. Siphonosteles also contain pith.  If the xylem is found only outside of the pith, it is called ectophloic. If the xylem can be found both within and outside of the pith, it is called amphiphloic. Members of the nightshade family, such as tomatoes and peppers, are amphiphloic. There are three types of amphiphloic steles:

• solenostele - this type has only one section of hula hoop cut away; fern stems are a common example
• dictyostele - this type has multiple sections cut from its hula hoop; another example of fern stems
• eustele - this is the Big Category of plants that contain actual vascular bundles that can be found in both roots and stems of monocot angiosperms

Types of siphonostele (EncycloPetey) Public Domain

Stele diseases

​Diseases of the stele include phytophthora root rot, verticillium wilt, black root rot, and crown rot. In each case, prolonged exposure to wet soil creates the conditions needed for pathogens to infect your plants. Maintaining good drainage and soil structure can help prevent these diseases.

So, why would you care what sort of stele your plants have? Besides sounding really smart, being able to look up information about what’s inside a plant stem can help you identify unknown plants.

​What's inside your stems?