THE ANGIOSPERMAE 1075 



realized that all forms of inflorescence merge into one another and that 

 mixed forms are common. Even the two main groupings into racemose and 

 cymose forms are not watertight. Whether or not one of them may have 

 been derived from the other in an evolutionary sense, they do show inter- 

 grades, and are not mutually exclusive categories. 



With these reservations we may proceed to the description of a number 

 of frequently recurring types, to which a large proportion of inflorescences 

 may be referred. 



By analogy with the vegetative branch system, it has been held that the 

 monopodial or racemose plan must be the primitive plan of inflorescence 

 branching, but that this is not necessarily true or even probable will be 

 perceived if we reflect that the formation of a terminal flower, that is to say 

 the original state of flowering in its simplest manifestation, brings the 

 growth of the axis to a stop and thus precludes any further development on 

 the monopodial plan. Any development beyond this initial condition will 

 therefore involve the production of lateral axes to replace the arrested apex, 

 that is to say it will be on the sympodial or cymose plan. A comparative 

 study of flower clusters shows that among Dicotyledons, the first stage of 

 cymose advancement is the development of two lateral axes beneath the 

 terminal flower, each in its turn ending in a flower and giving rise to another 

 pair of flower-bearing laterals, and so on. This is the dichasium. (This 

 and the following inflorescence types are illustrated diagrammatically in 

 Figs. 1044 and 1045.) 



Among Monocotyledons, it is commoner to find only one lateral formed 

 at each stage of branching, providing a monochasium. The same condi- 

 tion also occurs among Dicotyledons, but apparently as the result of reduc- 

 tion from two laterals to one. 



Monochasia present a variety of different appearances according to 

 their mode of development. When the successive laterals come off always 

 towards the same side of the axis we have a bostryx. The successive laterals 

 of a bostryx may stand at different angles to the parent axis. Not uncom- 

 monly the directions of successive laterals show regular angular deflections 

 either clockwise or anti-clockwise, building up a type of spiral inflorescence 

 to which some writers limit the application of the name. When the laterals 

 do in f£ct all lie in the same plane they produce a helicoid grouping known as 

 a drepanium. On the other hand, the laterals may alternate successively 

 towards opposite sides of the parent axis, thus building up a scorpioid 

 figure called a cincinnus. If the laterals of a cincinnus all lie in the same 

 plane so that they form a fan-shaped figure, they constitute a rhipidium, 

 chiefly a monocotyledonous type. The terms " helicoid " and " scorpioid " 

 have been much confused. In general both terms would apply descriptively 

 to a curled cymose inflorescence, such as that of Boraginaceae, but strictly 

 speaking " helicoid " should only be used of the bostryx or unilateral type 

 and " scorpioid " of the cincinnus or alternating type. 



If more than two laterals are formed at each stage of branching, we have 

 a pleiochasium, which is rare as a simple inflorescence but is not uncom- 



