182 THE БЕУ. С. HENSLOW ON THE ORIGIN OF FLORAL ASTIVATIONS. 
threes in these orders, I believe it to arise as just stated, and аз is more fully explained 
further on (Note C, р. 194) from phyllotactical considerations; that is, a prolonged 
spiral, instead of furnishing whorls corresponding to cycles, as in Aconitum, is simply 
broken up into decussating whorls of threes. 
3. Pentastichous or Quincuncial.—This arrangement is one in which the parts of а 
whorl constitute one cycle of that leaf-order. Each successive member is at an angular 
distance of 144° from the next in order. It appears to be the commonest in Dicotyledons, 
obtaining either in one, two, or more whorls of a flower, and, from the diagrams of 
Le Maout and Decaisne, occurs about 50 p. с. This deduction is quite corroborated by 
observations on individual plants; for in the flowers e.g. of Primrose % (which shows 
eight if not more varieties of æstivation) the pentastichous was about 40 p. с. Similarly 
is it with Viburnum tinus. 
What I have said upon the origin of alternate leaf-arrangements in my paper referred 
to above, fully accounts for this plan being the commonest among Dicotyledonous 
Angiosperms. Indeed I would venture to call it one of the fundamental plans, as 
giving rise to at least three others, if not more, such as the half-imbricate, imbricate 
proper, convolute, and possibly in part the equitant. 
4. Half-imbricate.—l apply this term to a very common arrangement, which does 
not appear to have been at all recognized before, but which is nevertheless a very 
important type of zstivation. It is deduced from the pentastichous by the 2nd member 
of that kind having one of its edges passing under the 4th, and is not only the first step 
towards the imbrieate proper, and thence to the convolute, but will also explain the 
estivation of many irregular flowers, such as the “ papilionaceous," that of the Cesalpinee, 
and the “ cochlear" t, which, too, can be thus accounted for by a simpler method than 
is usually applied, and moreover a perfectly natural one, as it exists abundantly amongst 
floral whorls. j 
In the percentages deduced from the diagrams of Le Maout and Decaisne, that of this 
kind does not appear very high (16 p. с.). Ап examination of many flowers leads one to 
think it should be somewhat higher, as, e. g., іп Laurustinus it amounts to nearly 
80 p. с., though in the Primrose it is certainly rare; similarly in Ribes it only amounts 
to about 10 p. c. 
With regard to the papilionaceous corolla, if we consider the standard as Хо. 1, one 
of the keel-petals as No. 2 (of a quincunx), which now passes under the wing, i. e. No. 4, 
the wing on the opposite side of the median line will be No. 3, while the remaining keel- 
petal will be No. 5, coherent with No. 2. 
* The diagram given by Le Maout and Decaisne of the Cowslip (miscalled Primrose), on p. 529, has both the calyx 
and corolla according to the imbricate-proper sestivation, and not pentastichous, which is much the most characteristic 
form. See Note D, p. 195. 
Since this paper was sent to press, my attention has been called to Dr. A. W. Eichler's * Blüthendiagramme, in 
which he observes that this form is found both in regular and irregular flowers, and he gives a diagram of it for 
Adnan, р. 270, and Valeriana officinalis, p. 275 ; but һе retains the name “ cochlear." This I purposely avoided, 28 
it has only a specific and not generic value. 
+ In the translation of Le Maout and Decaisne the word “ cochleate” is used instead of “ cochlear," which is the 
correct term : the former is descriptive of a spiral shell, and would be applicable to the coiled legume of Medicago. 
