A plant root is an organ, and last week we looked at the specialised regions along a root in a longitudinal (lengthwise) section. Today we go in at right angles and examine what is revealed by a cross-sectional cut across a root.
Before doing so, a very quick up-to-speed on the the classification of flowering plants — of which of course the jujube tree is one — as this is actually relevant!
Flowering plants are the angiosperms (from Ancient Greek: ἀγγεῖον, angeion, case or receptacle, and σπέρμα, sperma, seed — seeds enclosed by a fruit). All flowering plants produce fruits containing seeds, though not all fruits are recognisable as such to the unfamiliar eye. Caraway ’seeds’, for example, are really a dry fruit which contains the real caraway seed.
Angiosperms can be divided into two groups: the monocotyledons (also known as monocots) and the dicotyledons (also known as dicots).
Monocots have a single coteyledon (embryonic, first, leaf), and dicots have two. Monocots have other characteristics such as a fibrous root system and leaf veins in a parallel arrangement — think grasses.
Dicots, on the other hand, have a tap root system and leaf veins in a network (reticulated) arrangement, and yes, the jujube tree is a dicot.
Now the relevant part: while monocots and dicots have the same tissues visible in a root cross-section, these are arranged differently in monocots compared to dicots. (Dicot roots have an X-shape in the middle, while monocots have a ring instead.) I will be concentrating only on dicots in this discussion.
It would take me a week to draw something halfway decent and I don’t have a camera for my microscope (I really should think about getting one…!), so may I refer you to these excellent diagrams instead and invite you to follow along!
Let’s back-track briefly to last week and revisit the division zone immediately behind the root cap. This division zone is also called the apical (of the apex) meristem, though not at the very tip of the root (because of the root cap) as is the stem apical meristem at the very tip of a stem.
Bear in mind that last week dealt with the lengthwise distribution of specialised root cells. When reading the below, please also refer to the above-linked cross-section (also here) and visualise the two combined into a 3D structure.
Root Apical Meristem
As with the stem apical meristem, the root apical meristem is also where cell division and differentiation occurs. The apical root meristem further differentiates into three other meristems: the protoderm, which becomes the epidermis; the procambium, which becomes the vascular cylinder, or stele; and the ground meristem, which becomes the cortex and endodermis.
The protoderm meristem forms the epidermis, an outer layer of protective cells which also aid in absorption of water and nutrients. The root epidermis is also known as the rhizodermis. It is these cells in the maturation zone which produce the extensions that become root hairs.
The procambium meristem develops into the vascular (transport) cylinder, also called the stele, located at the centre of the root. The stele is comprised of xylem, phloem and pericycle tissues.
The xylem and phloem together are called the vascular (transport) bundle, and it’s the xylem which forms the distinctive X-shape in a dicot root cross-section.
Xylem transports water and dissolved nutrients from the roots up to the stems and leaves, and phloem transports food produced from photosynthesis (photosynthates) from the leaves down to the non-photosynthesising stems and roots of a plant.
The pericycle strengthens the roots, protects the vascular bundle, and can later develop into lateral roots.
The ground-meristem forms both the cortex, which is the region between the epidermis and stele, and the specialised endodermal layer which surrounds the stele. This layer is exclusive to roots and controls which substances can and cannot enter the stele — similar in a way to our own blood-brain barrier.
A waxy region on the endodermal cell walls called the Casparian strip forces water and water-soluble nutrients to pass through the endodermis rather than slipping between the cell walls. This same strip also excludes pathogens and toxins.
Having explored general root anatomy, next week we’ll cover what makes some roots produce suckers!
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