THE FLOWER (AESTIVATION) 
75 
are closely packed in a definite way which is always the 
same for the same flower and often for a whole family ; so 
that this aestivation (ef. vernation, p. 41) is a character of 
importance in classification. Only the perianth is con- 
sidered, as a rule, and the aestivation is easily seen in a cross 
section (cf. floral diagrams in Part II). 
Descriptive Terms, <Srv. Leaves or segments not even meeting at 
edges, the aestivation is open (corolla of Cruciferae), touching but not 
overlapping, valvate (corolla of Compositae), overlapping, imbricate 
(calyx of Leguminosae). Special cases of imbricate are convolute or 
contorted (each leaf overlapping with the same right or left edge, so that 
the corolla looks twisted, as in Malva, Gentiana, &c.) and quincuncial 
(two leaves overlapping with both edges, two underlapping with both, 
one over- and under-lapping, as in calyx of Caryophyllaceae). Each 
leaf overlapping the one posterior to it, ascending (calyx of Vicia in 
Leguminosae), anterior to it, descending (corolla of Vicia). Leaf 
margins turned inwards, induplicate (corolla of many Compositae), 
outwards, reduplicate. Leaves rolled up inwards like watch springs, 
cuxinate (petals of Hamamelidaceae). 
Most commonly all the flowers on a plant show the same aestivation 
in detail, or are homodromous, as opposed to the heterodromous flowers 
of Marantaceae, Exacum, Saintpaulia &c., in which a right-hand 
flower is arranged the opposite way to a left-hand flower, so that a 
particular organ twisted to the right in one is twisted to the left in the 
other. 
The Perianth , or outer covering of the flower, Com- 
posed of non-reproductive leaves, is an organ that dates its 
origin very far back, as it now occurs in the vast majority 
of flowers ; a few naked flowers exist, but in studying them 
we are at once met with the question, “ Are they primitively 
naked (i.e. have all their ancestral forms, however far they 
be traced, also been naked) or are they naked by reduction, 
i.e. by the loss of a perianth possessed at some period by 
their ancestry ? ” Those naked flowers which occur among 
the Chalazogamae are probably primitively so, their other 
characters attesting their antiquity, and perhaps the Gra- 
mineae and some of the lower Dicotyledons may also be 
included here, but such flowers as Achlys, Euphorbia, 
Altinga, &c. are certainly naked by suppression, occurring 
as they do in families most of whose flowers are not naked. 
The origin of the perianth is a subject beset with 
difficulty ; there are various explanations possible. It may 
have been derived from true vegetative leaves which gradu- 
ally grew closer to the stamens and carpels, or from 
