442 MORPHOLOGY OF THE ANGIOSPERMS 



ovules are chiefly in submarginal rows, with traces from the ventral 

 bundles, but some ovules derive their traces from the laminar meshwork 

 between the median and lateral veins. This vascularization is evidence 

 that the submarginal placentation in the Liliaceae represents reduction 

 from laminar, as is shown for the dicotyledons in the Degeneriaceae and 

 Winteraceae. The laminar placentation, as seen in the Helobiales 

 {Butomus), is reduced to submarginal in the Liliaceae and to basal 

 (one or two ovules) in Alismatales (Scheiichzeria). The position of the 

 solitary ovule of Aphi/Uanfhcs (Liliaceae) is described, obscurely, as 

 "marginal median," perhaps between the median and lateral veins, 

 as in Cabomba. The story of placentation reduction is seen in both 

 dicotyledons and monocotyledons. 



PALMAE 



The palms, one of the largest families of the monocotyledons — over 

 two hundred genera and four thousand to five thousand species, are 

 remarkable for range in form and for combinations of primitive and 

 advanced characters. Unfortunately, the family is not well known mor- 

 phologically, because of its large size and the difficulty of obtaining 

 material. There has been comparatively little anatomical study of the 

 flowers and inflorescences. 



Habit 



In habit, the palms range from small, essentially herbaceous — acaules- 

 cent — types to large, unbranched trees — some, 100 feet tall — and there 

 are woody vines and clustered, much-branched "shrubs." Many of the 

 arborescent taxa branch occasionally. The great range in habit, to- 

 gether with anatomical structure, suggests that the arborescent, un- 

 branched habit has been derived from the herbaceous, rhizomatous, 

 freely branching habit. (The bamboos seem to represent a similar state 

 in the grasses.) 



The trunks of many palm trees consist entii'ely of primary tissues, 

 built up by the huge, terminal meristem; in other genera, there is in- 

 crease in trunk diameter by secondary growth. This increase in diameter 

 is acquired after increase in length of the region has ceased and, 

 histologically, resembles primary growth rather than that brought 

 about by a cambium. New-cell formation in the inner cortex adds new 

 tissues — vascular bundles, fiber strands, and parenchyma, which re- 

 semble the central primary tissues. (A small amount of "delayed" 

 primary increase in diameter occurs in some palms after the tightly 

 sheathing leaf base is loosened or lost. This increase consists of en- 



