Roots
Functions
Anchoring the plant to the soil
Absorption of water and minerals
Production of Hormones
Allows the rest of the plant to be properly oriented towards the sun
Without roots, trees and shrubs would simply fall over
Cylindrical shape of roots allow for equal absorption on all sides
Roots play a large role in the production of the hormones that initiate shoot growth
Selectively advantageous because the leaves that grow from the shoot should not transpire more than the roots absorb, which would be a waste of energy
Some roots are used for carbohydrate storage
Ex. carrots, beets, radishes
Some roots can grow horizontally and sprout functionally new plants
Some plants use their roots to sap water and nutrients from other plants
Form of Parasitism
External Structure of Roots 
Taproot
Main, Central Root
Branch Roots
Also called lateral roots
Smaller roots coming out of the taproot
Develops from the embryonic root
Present in the seed
Usually the largest root
Can produce lateral roots of their own
Fibrous Root System
Root system in which doesn't include a radicle. Instead, all of the roots originated from stem tissue
Common in monocots and eudicots
Radicle dies during or after germination in this case
Fibrous root systems are made up of adventitious roots
Adventitious roots do not come from pre-existing roots nor are they radicles
Adventitious roots increase the absorptive and transport capabilities of the root system
Many monocots don't have a taproot because they cannot undergo secondary growth
Individual Roots
Fairly Simple
Root tip is where growth occurs
Roots grow only by discrete apical meristems
Result is localized growth, which means only one area grows at a time
The root apical meristems are protected by a thick layer of cells
This is because soil has dangerous particles on a microscopic level for something as sensitive as an apical meristem
This thick layer is called the root cap
The root cap is constantly worn away and must be reinforced by more cells
The golgi apparatus of root cap cells produces a polysaccharide called mucigel that makes it easier for the root to dig into the soil
Behind the root cap is the zone of elongation
Expansion occurs here
Very short, at three millimeters long
Behind the zone of elongation is the root hair zone
This is where many epidermal cells sprout outward as trichomes
In case we forgot what trichomes are, they are the most interesting part of the plant in my most unbiased opinion
They are essentially the plant's weapon arsenal
Only form in parts of the root that isn't elongating
For some reason, they appear here to aid in absorption of water and minerals
Specifically, they are able to crawl their ways into tinier crevices than the roots, and gain access to valuable nutrients in that fashion
They can also make themselves useful underground without anything to stab by respiring to give off carbon dioxide
This carbon dioxide then combines with soil water to form carbonic acid, which further helps make the soil more permeable for the root
Teamwork makes the dream work
Internal Structure of Roots
Root Cap
Cells grow through the edges of the root cap
Cells first formed at the base of the root cap are small but are constantly dividing
These cells develop dense starch grains and their endoplasmic reticulum shifts towards the front of the cell
This process allows the cells to detect gravity
As these cells get closer to the edge of the root cap, their endoplasmic reticulum shrinks, their starch grains are digested, and their golgi body produces large amounts of mucigel
Root Apical Meristem
Uniform pattern of cells
This is due to no irregularities, unlike the shoot
The center does not experience cell division
This area is called the quiescent center
It is believed this area is a reserve of healthy cells due to their relatively strong resistance to harmful agents
When a part of the root is seriously damaged, the quiescent center becomes active and reestablishes a new apical meristem
The center of those cells then becomes the new quiescent center
Zone of Elongation
In this area there are some meristematic cells but most cells are enlarging
The outer cells differentiate into epidermal cells
The central cells differentiate into the xylem and phloem
The closer cells are to the root cap, the younger they are, and vice versa
Permeable enough for minerals to enter by diffusion
Too small to play a large role in absorption
Root Hair Zone
Contains root hairs, which aid in absorption
No distinct boundary between the root hair zone and zone of elongation
Minerals have a hard time getting to the central vascular tissue due to the endodermis
The walls of the endodermis has different qualities depending on the area it is found
The walls closest to to the cortex and vascular tissue are not too different from average primary walls
The walls on the borders of other endodermis cells are waterproof
Along the entirety of the endodermis, these walls form a layer within the endodermis cells called the Casparian strips
This is the specific part of the endodermis that controls what minerals enter the xylem water stream
Basic location of cells in the root hair zone
Cortex
A mass of mostly parenchyma cells between the epidermis and endodermis
Epidermis
The outer layer of dermal cells
Endodermis
Constitutes most of the space in the root hair zone
Vascular Tissue
The inner layer of dermal cells
Cells enclosed within the endodermis
Without the endodermis, any mineral inside the intercellular space could move into the xylem
The Casparian strips are also impermeable
Due to this, only minerals that are absorbed by endodermal protoplasts make it into the vascular tissue
The vascular tissue is only in any real danger of unwanted minerals when the endodermis is immature
Vascular Tissue
In some monocots, strands of xylem and phloem are distributed in ground tissue within the endodermis
Xylem forms a mass in the center. Surrounding the mass of xylem are strands of phloem
This is the arrangement of the vascular tissue in most plants except some monocots
Besides differences in arrangement, vascular tissue of the root hair zone is akin to those found in the stem and leaf
The cells enclosed in the endodermis that aren't xylem or phloem are collectively called the pericycle
The pericycle is made up of parenchyma cells
Lateral roots sprout from the pericycle
Lifespan for individual root hairs is only a few days
Some endodermis cells consist of only Casparian Strips
These cells are called passage cells due to a past misunderstanding
Currently thought to be underdeveloped cells
Water pressure, called root pressure, builds up due to root hair's absorptive efficiency
The strong endodermis prevents the root pressure from resulting in a leakage of water
Origin and Development of Lateral Roots
Lateral roots originate from the pericycle found within the endodermis
As the root primordium develops, it organizes itself into a root apical meristem and begins growing into the cortex
As this happens, the endodermis will either be torn or create a thin layer over the primordium
Ultimately, this will result in the destruction of the endodermis
By the time the root primordium makes it's way out of the parent root, it would already have it's own root cap
The rest of the process continues as it did for the parent root
Other Types of Roots and Root Modifications
Storage Roots
These roots accumulate excess carbohydrates from summertime photosynthesis
During Autumn, some plants will die save for the taproot, where the carbohydrates are stored
When Spring comes, these plants produce a new shoot from the carbohydrates stored
Somewhat similar to hibernation in animals
Plants that don't die out during autumn may still use storage roots for the less productive period during winter
Prop Roots
Prop roots are roots that exist to stabilize the stem of the plant by applying slight tension on it by use of contractions
The length of a prop root can be meters long, and growth can take many months
Allows for the growth of massive trees
Also effective for plants growing beside a body of water due to it's brace-like structure
One example of this is the mangrove
These roots absorb minerals through the air to be transported to the subterranean roots, due to the unfavorable soil conditions
Aerial Roots of Orchids
Roots that dangle freely in the air
Orchid is wrapped around the branch of a tree
Orchids must have water conservation mechanisms to avoid the absorption of water by their surroundings
The root epidermis, called a velamen in orchids, is composed of many layers of dead cells that act as a waterproof barrier between the orchid's roots and it's surroundings
Contractile Roots
Contract even more than prop roots
Follows the same path of prop roots, but pulls the stem down until the base of the shoot is at soil level or below
The contractions come from the cortex cells
Provides a way for the plant to manually adjust depth
This is helpful in cases where seeds germinate at soil level or for subterranean stems
Mycorrhizae
The symbiotic relationships between the vast majority of seed plants and soil fungi
Ectomycorrhizal Relationship
Fungal hyphae penetrate the outermost part of the cortex but never actually invades the cells
Endomycorrhizal Association
Hyphae pass all the way into the root cells to right before the endodermis
Includes the invasion of cells but doesn't result in the destruction of the host plasma membrane or vacuole membrane
However the fungi make their way into the root, the end result is the exchange of sugars for phosphorus by the plant
Root Nodules and Nitrogen Fixation
Plants do not have the enzyme systems necessary to use nitrogen from the air
Plants therefore rely on different methods to acquire necessary nitrogen
Some prokaryotes that use nitrogen by incorporating them as amino acids and nucleotides can be used by plants to obtain nitrogen when they die
The process the prokaryotes use to convert atmospheric nitrogen into a usable form is called nitrogen fixation
Nitrogen-producing bacteria of genus Rhizobium enter the roots of a small number of plants by way of an infection thread
When inside, the bacteria multiplies rapidly
Entry of the bacteria is through the root hairs
This fills the plant cells with bacteria capable of converting nitrogen into usable compounds
The plant cells provide the sugars required for the process, and both parties benefit
During this stage, the bacterial enzymes can be damaged by oxygen.
The plant produces a chemical called leghemoglobin that binds to oxygen and keeps the enzymes safe
Must be noted that this process isn't necessarily voluntary. Both parties are just acting in the manner that led to their forerunner's survival
Haustorial Roots of Parasitic Flowering Plants
Highly modified roots of parasitic angiosperms
Strikingly different from normal roots on a structural level
Loosely defined, but will usually stick close to the host by way of adhesive secretion or growing around a part of the plant
To penetrate the dermal system of the host, the Haustorial Roots force a shaft of cells through or crushes it by expansion of the haustorium
The parasitic cells then make contact with the host xylem, and no other cells
The result of this is a vessel consisting of both host and parasite cells
The parasite then conducts it's own photosynthesis
Roots of Strangler Figs
Parasitic Trees
Fruit of fully grown strangler fig is eaten by a bird, then it deposits the seed on another tree
The seed germinates. It's roots spread down the length of the host tree, sustaining itself from rainwater for months or years
Upon reaching soil, the roots grow quickly underground while those still above ground grow woody. The strangler fig soon encases the host tree
Eventually the host tree dies from being shaded or due to stunted growth, and the strangler fig becomes a self-supporting tree