This article may be too technical for most readers to understand.(August 2017)
A raceme ( // or //) or racemoid is an unbranched, indeterminate type of inflorescence bearing pedicellate flowers (flowers having short floral stalks called pedicels) along its axis.  In botany, an axis means a shoot, in this case one bearing the flowers. In indeterminate inflorescence-like racemes, the oldest flowers are borne towards the base and new flowers are produced as the shoot grows, with no predetermined growth limit.  A plant that flowers on a showy raceme may have this reflected in its scientific name, e.g. Cimicifuga racemosa. A compound raceme, also called a panicle, has a branching main axis.  Examples of racemes occur on mustard (genus Brassica) and radish (genus Raphanus) plants. 
A spike is an unbranched, indeterminate inflorescence, similar to a raceme, but bearing sessile flowers (sessile flowers are attached directly, without stalks).  Examples occur on Malabar nut ( Justicia adhatoda) and chaff flowers (genus Achyranthes).  A spikelet can refer to a small spike, although it is primarily used to refer to the ultimate flower cluster unit in grasses ( family Poaceae) and sedges (family Cyperaceae),  in which case the stalk supporting the cluster becomes the pedicel. A true spikelet comprises one or more florets enclosed by two glumes (sterile bracts), with flowers and glumes arranged in two opposite rows along the spikelet.  Examples occur on rice (species Oryza sativa) and wheat (genus Triticum), both grasses. 
An ament or catkin is very similar to a spike or raceme, "but with subtending bracts so conspicuous as to conceal the flowers until pollination, as in the pussy–willow, alder, [and] birch...".  These are sometimes called amentaceous plants. 
A spadix is a form of spike in which the florets are densely crowded along a fleshy axis, and enclosed by one or more large, brightly–colored bracts called spathes.  Usually the female flowers grow at the base, and male flowers grow above.  They are a characteristic of the family Araceae, for example jack–in–the–pulpit (species Arisaema triphyllum) and wild calla (genus Calla). 
Every radiating unit in this inflorescence of a Cyperus sedge is a spikelet composed of small flowers (florets) arranged in two ranks.
From classical Latin racemus, cluster of grapes. 
- Walters, Dirk R.; Keil, David J. (1 January 1996). Vascular Plant Taxonomy (4th ed.). United States: Kendall/Hunt Publishing Company. p. 602. ISBN 978-0-7872-2108-9.
- Wofford, B. Eugene (1989). Guide to the Vascular Plants of the Blue Ridge. University of Georgia Press. pp. 10–15. ISBN 978-0-8203-2455-5.
- Kumar, Vinay; Bhatia, S. S. (2013). Complete Biology for Medical College Entrance Examination. McGraw Hill Education Series (3rd ed.). McGraw Hill Education (India) Private Limited. p. 218. ISBN 978-1-259-06430-2.
- Gilman, Daniel Coit., ed. (1907). The new international encyclopædia. 10. Peck, Harry Thurston; Colby, Frank Moore. New York: Dodd, Mead and Company. p. 618.
- Oxford English Dictionary. Raceme 2. Bot. A type of inflorescence in which the flowers are arranged on short, nearly equal, lateral pedicels, at equal distances along a single elongated axis
|Wikimedia Commons has media related to Raceme.|