308 SYMMETRY OF THE FLOWER.—TRANSVERSE CHORISIS. 
tion in the fact, that it not only increases the number of parts, 
but also interferes with their regular alternation ; for augment- 
ation does not necessarily interfere with alternation, it only 
obscures it when the number of additional parts is excessive, or 
when the whorls are crowded together. 
Chorisis may take place in two ways, either transversely, 
when the increased parts are placed one before the other, which 
is called vertical, parallel, or transverse chorisis ; or collaterally, 
when the increased parts stand side by side, which is termed 
collateral chorisis. Transverse chorisis is supposed to be of fre- 
quent occurrence ; thus the petals of Lychiis (fig. 501, a) and 
many other Caryophyllaceous plants, exhibit a little scale on 
their inner surface at the point where the limb of the petal is 
united to the claw. A somewhat similar scale, although less 
developed, occurs at the base of the petals of some species of 
Ranwnculus (fig. 498). The formation of these scales is supposed 
by many to be due to the chorisis or unlining of an inner por- 
tion of the petal from the outer. Other botanists consider these 
appendages as abortive stamens, or glands (see page 240). Hach 
petal of Parnassia (fig. 500) has at its base a petal-like append- 
age divided into a number of parts, somewhat resembling sterile 
stamens ; this is also stated to be produced by transverse cho- 
risis. 
In, plants of the orders Rhamnacez (fig. 793), and others, 
the stamens are placed opposite to the petals, hence they are 
supposed by many botanists to be produced 
Fic. 793. by chorisis from the corolla; but others 
explain this opposition of parts by sup- 
posing the suppression of an intermediate 
whorl (see page 360). Transverse chorisis 
is also frequently to be found in the 
andrceecium, but it is less frequent in the 
gyncecium. Examples of transverse chorisis 
in the gyncecium are furnished, however, by 
Crassula (fig. 785), where each carpel has 
Fig. 793. Diagram of at its base on the outside a little greenish 
Ee OW er cy, scale, a, a, which is supposed by some to 
articus), be due to it. 
It will be observed, that in the above 
cases of transverse chorisis, the parts which are produced do 
not resemble those from which they arise, and this appears to 
be a universal law in this form of chorisis. 
Collateral Chorisis.—We have a good example of this form in 
the Stock, Wallflower, and other plants of the order Crucifere. 
In these flowers, the two floral envelopes are each composed 
of four organs alternating with one another (jig. 794). Within 
these we find six stamens, instead of four, as should be the 
case in a symmetrical flower ; of these two are placed opposite 
to the lateral sepals and alternate with the adjacent petals, 
