GAF Timberline Natural Shadow Charcoal Algae Resistant Architectural Shingles (33.33 sq. ft. per Bundle) (21-Pieces)

Twenty-one, XXI or 21 may refer to:

• 21 (number), the natural number following 20 and preceding 22
• The years 21 BC, AD 21, 1921, 2021

Algae (UK: AL-ghee, US: AL-jee; sg.: alga AL-gə) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular microalgae, such as Chlorella, Prototheca and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 metres (160 ft) in length. Most are aquatic and lack many of the distinct cell and tissue types, such as stomata, xylem and phloem that are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and stoneworts. Algae that are carried by water are plankton, specifically phytoplankton.

Algae constitute a polyphyletic group since they do not include a common ancestor, and although their plastids seem to have a single origin, from cyanobacteria, they were acquired in different ways. Green algae are examples of algae that have primary chloroplasts derived from endosymbiotic cyanobacteria. Diatoms and brown algae are examples of algae with secondary chloroplasts derived from an endosymbiotic red alga. Algae exhibit a wide range of reproductive strategies, from simple asexual cell division to complex forms of sexual reproduction.

Algae lack the various structures that characterize land plants, such as the phyllids (leaf-like structures) of bryophytes, rhizoids of non-vascular plants, and the roots, leaves, and other organs found in tracheophytes (vascular plants). Most are phototrophic, although some are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species of green algae, many golden algae, euglenids, dinoflagellates, and other algae have become heterotrophs (also called colorless or apochlorotic algae), sometimes parasitic, relying entirely on external energy sources and have limited or no photosynthetic apparatus. Some other heterotrophic organisms, such as the apicomplexans, are also derived from cells whose ancestors possessed plastids, but are not traditionally considered as algae. Algae have photosynthetic machinery ultimately derived from cyanobacteria that produce oxygen as a by-product of photosynthesis, unlike other photosynthetic bacteria such as purple and green sulfur bacteria. Fossilized filamentous algae from the Vindhya basin have been dated back to 1.6 to 1.7 billion years ago.

Because of the wide range of types of algae, they have increasing different industrial and traditional applications in human society. Traditional seaweed farming practices have existed for thousands of years and have strong traditions in East Asia food cultures. More modern algaculture applications extend the food traditions for other applications include cattle feed, using algae for bioremediation or pollution control, transforming sunlight into algae fuels or other chemicals used in industrial processes, and in medical and scientific applications. A 2020 review found that these applications of algae could play an important role in carbon sequestration in order to mitigate climate change while providing lucrative value-added products for global economies.

Bundle or Bundling may refer to:

• Bundling (packaging), the process of using straps to bundle up items

Charcoal is a lightweight black carbon residue produced by strongly heating wood (or other animal and plant materials) in minimal oxygen to remove all water and volatile constituents. In the traditional version of this pyrolysis process, called charcoal burning, often by forming a charcoal kiln, the heat is supplied by burning part of the starting material itself, with a limited supply of oxygen. The material can also be heated in a closed retort. Modern charcoal briquettes used for outdoor cooking may contain many other additives, e.g. coal.

The history of wood charcoal production spans ancient times, rooted in the abundance of wood in various regions. The process typically involves stacking wood billets to form a conical pile, allowing air to enter through openings at the bottom, and igniting the pile gradually. Charcoal burners, skilled professionals tasked with managing the delicate operation, often lived in isolation to tend their wood piles. Throughout history, the extensive production of charcoal has been a significant contributor to deforestation, particularly in regions like Central Europe. However, various management practices, such as coppicing, aimed to maintain a steady supply of wood for charcoal production. The scarcity of easily accessible wood resources eventually led to the transition to fossil fuel equivalents like coal.

Modern methods of charcoal production involve carbonizing wood in retorts, yielding higher efficiencies compared to traditional kilning methods. The properties of charcoal depend on factors such as the material charred and the temperature of carbonization.

Charcoal finds diverse applications, including metallurgical fuel in iron and steel production, industrial fuel, cooking and heating fuel, reducing agent in chemical processes, and as a raw material in pyrotechnics. It is also utilized in cosmetics, horticulture, animal husbandry, medicine, and environmental sustainability efforts, such as carbon sequestration.

However, the production and utilization of charcoal can have adverse environmental impacts, including deforestation and emissions. Illegal and unregulated charcoal production, particularly in regions like South America and Africa, poses significant challenges to environmental conservation efforts.

Per is a Latin preposition which means "through" or "for each", as in per capita.

Per or PER may also refer to:

A shadow is a dark area where light from a light source is blocked by an object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette, or a reverse projection of the object blocking the light.

Shingles, also known as herpes zoster, is a viral disease characterized by a painful skin rash with blisters in a localized area. Typically the rash occurs in a single, wide mark either on the left or right side of the body or face. Two to four days before the rash occurs there may be tingling or local pain in the area. Other common symptoms are fever, headache, and tiredness. The rash usually heals within two to four weeks; however, some people develop ongoing nerve pain which can last for months or years, a condition called postherpetic neuralgia (PHN). In those with poor immune function the rash may occur widely. If the rash involves the eye, vision loss may occur.

Shingles is caused by the varicella zoster virus (VZV) that also causes chickenpox. In the case of chickenpox, also called varicella, the initial infection with the virus typically occurs during childhood or adolescence. Once the chickenpox has resolved, the virus can remain dormant (inactive) in human nerve cells (dorsal root ganglia or cranial nerves) for years or decades, after which it may reactivate and travel along nerve bodies to nerve endings in the skin, producing blisters. During an outbreak of shingles, exposure to the varicella virus found in shingles blisters can cause chickenpox in someone who has not yet had chickenpox, although that person will not suffer from shingles, at least on the first infection. How the virus remains dormant in the body or subsequently re-activates is not well understood.

The disease has been recognized since ancient times. Risk factors for reactivation of the dormant virus include old age, poor immune function, and having contracted chickenpox before 18 months of age. Diagnosis is typically based on the signs and symptoms presented. Varicella zoster virus is not the same as herpes simplex virus, although they belong to the same family of herpesviruses.

Shingles vaccines reduce the risk of shingles by 50 to 90%, depending on the vaccine used. Vaccination also decreases rates of postherpetic neuralgia, and, if shingles occurs, its severity. If shingles develops, antiviral medications such as aciclovir can reduce the severity and duration of disease if started within 72 hours of the appearance of the rash. Evidence does not show a significant effect of antivirals or steroids on rates of postherpetic neuralgia. Paracetamol, NSAIDs, or opioids may be used to help with acute pain.

It is estimated that about a third of people develop shingles at some point in their lives. While shingles is more common among older people, children may also get the disease. According to the US National Institutes of Health, the number of new cases per year ranges from 1.2 to 3.4 per 1,000 person-years among healthy individuals to 3.9 to 11.8 per 1,000 person-years among those older than 65 years of age. About half of those living to age 85 will have at least one attack, and fewer than 5% will have more than one attack. Although symptoms can be severe, risk of death is very low: 0.28 to 0.69 deaths per million.

Timberline is an alternate term for the tree line.

It may also refer to: