THE ANGIOSPERMAE 1079 



however immature, but in practice it may be impossible to distinguish 

 whether the undeveloped apex of the inflorescence was or was not a terminal 

 flower, and in such cases we must allow ourselves to be guided by the 

 acropetal order of flowering and place all such types under the monopodial 

 heading. 



A third method of evolution may have been through the indefinite 

 prolongation of intercalary inflorescences, the growing point of the shoot 

 continuing to produce bracts with axillary flowers, instead of reverting to 

 vegetative leaves as is usual in such inflorescences. 



The simplest type of monopodial inflorescence is the raceme, in which 

 single flowers are borne on stalks in the axils of spirally arranged or whorled 

 bracts. The flowers normally open in acropetal succession and the apex is 

 either an indefinitely active growing point or else is abortive. A similar 

 arrangement, but with sessile flowers is the spike, for example the inflores- 

 cence of Plant ago, or the " catkins " of Salix. Where no internodes are 

 developed these two types give rise respectively to: the umbel, with stalked 

 flowers, arising from a common node; or the capitulum, with sessile 

 flowers closely aggregated upon a stumpy, contracted, or even flattened 

 axis.* Umbels may be simple or compound. In the latter each primary 

 stalk bears a separate small umbel. 



These are all relatively simple forms and where the acropetal order of 

 flowering exists are generally recognizable. 



The last of the racemose types is produced by the extra-elongation of 

 the lower flower-stalks, which raises the flowers of the whole raceme to a 

 common level at the top of the inflorescence. This is the corymb, a 

 common type among the Cruciferae. It corresponds to the cymose rhipi- 

 dium. The bracts in Cruciferae are frequently suppressed and the corymbs 

 especially are usually without them. 



It must be pointed out, indeed, that both the umbel and the corymb 

 may be simulated by cymose as well as racemose flower groups. The order 

 of flowering may or may not serve to distinguish them and in practice it 

 does not seem desirable to attempt a distinction. 



As we remarked above we have no direct information about the evolu- 

 tion of inflorescences, but we can see on biological grounds that some types 

 oflFer advantages over others and we may conclude from this that they are 

 probably more advanced. For example, compound types like the panicle 

 afford a longer flowering period, and other things being equal this offers 

 better opportunities for pollination, less subject to the drawbacks of tem- 

 porary bad weather or accidental damage. The simple raceme may afford 

 a similar advantage if its growth is truly monopodial and prolonged, with, 

 in addition, some economy of material and closer proximity of the indivi- 

 dual flowers. This latter point has often been indicated as an advantage 

 in the condensed types, such as the capitulum, umbel, etc., but the gain in 

 conspicuousness which, it is claimed, is thus attained by close massing, 



* The expanded basis of a capitulum is usually called the receptacle, but to avoid confusion 

 with the receptacle of the individual flowers the term pJioranth is to be preferred. 



