# Biological Importance of Cotton By-Products Relative to Chemical Constituents of the Cotton Plant
## 1 Introduction  
Cotton (_Gossypium_) is naturally a perennial plant that is now commercially cultivated as an annual plant in many parts of the world [1]. The cotton bud is the most utilized part of the plant and is the starting raw material for a wide range of products, such as textiles, edible oil, paper, livestock feed, and medicinal products, to name a few [2; 3; 4; 5; 6; 7]. Cotton fibre has many positive characteristics (comfort, colour retention, absorbency, strength) [2] and, hence, global cultivation has increased to an estimated production of over 23 million tonnes in 2013-2014 [8]. This increase in cotton production has resulted in tonnes of waste remaining after harvesting and processing (ginning), which has contributed to a growing challenge of its disposal [9; 10].  
Non-cotton fibre biomass residues generated from cotton production and processing includes cotton gin trash (CGT), post-harvest field thrash (PHT), and crushed seeds from which oil has been extracted. Post-harvest trash (PHT) are the remaining parts of the plant left on the field, while CGT is centralised at gins and is comprised mainly of sticks, burrs (calyx), leaves, and soil [9; 11]. These by-products of the cotton industry, although underutilized, are being used as soil composts and cottonseed meal nutritional supplements for livestock feed [12; 10; 13; 14]. Other methods of utilizationof cotton industry by-products have been explored, particularly as a low-cost biomass feedstock for commercial bioenergy/biofuel applications [15; 16; 17], mostly from CGT.  
The entire cotton plant has the potential to be a source of valuable compounds, such as terpen, phenolics, fatty acids, lipids, carbohydrates, and proteins [18; 19; 20; 21; 22; 23]. These compounds, which are distributed in seeds, bolls, calyx, leaves, stalks, stems, and roots of the plant [20; 23; 24; 25] play functional biological roles in humans and animals [26; 27; 28; 29; 21]. Gossypol, a poly-phenolic with potential contraceptive effects [30] and trans-caryophyllene, a terpenoid having anti-inflammatory and cytotoxic properties [31; 32], are examples of compounds present in cotton with potential beneficial impact on humans and animals. Thus, by-products generated by the cotton industry (CGT, PHT, and crushed seeds) may represent a potential source of valuable extractives due to the distribution of chemical compounds throughout the whole cotton plant.  
Agricultural products and by-products other than cotton are being exploited for generation of energy and materials [33], serving as a means of recycling and reducing organic wastes in the environment. Biomass from soybean, rice, and sugar cane are examples of agricultural by-products currently utilized for these purposes [34; 35; 36; 37]. Valuable chemical extractives may also occur in some of this agricultural biomass [38], although these potential resources have not been fully exploited.  
The multitude of potentially valuable chemicals that could be derived from cotton production by-products has gained little attention. Hence, exploiting the full potential of cotton waste will be beneficial to both the cotton industry and the local environment. A more detailed investigation is necessary to provide answers to such questions as: what is the nature of the extractives present in these by-products; how do the chemical profiles vary between cotton species and/or varieties and what potential uses do the waste products have as a source of high value compounds. To provide answers to some of these questions, this review begins with an overview of cotton production, the source of industry wastes, and its current utility. This review continues with a further discussion of current knowledge of chemical extractives present in the cotton plant and the distribution of these compounds within the plant highlighting their potential phytochemical properties which may be of value to both the pharmaceutical and agricultural sector.