The External Form of ‘Plants. Oe 
the involucre of Umbelliferse [and Composite], are different 
forms of bracts. 
To this class belong also the scade- or cataphalaty Leaves, 
which are either colourless, yellowish, reddish, brown, or 
rarely green; often fleshy, leathery, or membranous, and 
which for the most part remain buried in the ground. When 
the true foliage-leaves are strongly developed, the scale- 
leaves are generally inconspicuous ; while in other plants 
they are conspicuous, as in saprophytes which are desti- 
tute of chlorophyll and have no true leaves. 
The foliage-leaves—often called simply leaves—are dlis- 
tributed over the stem in a great variety of ways. When 
there is more than one on the same level they are opposite, 
decussate (Fig. 138), or verticillate (Fig. 139) ; when there is 
only ene on the same level, alternate or scattered (see p. 76). 
Irregular as the arrangement appears in the last case, a 
close investigation nevertheless shows that these also are 
distributed on-the stem in accordance with definite laws. If 
a spiral is drawn round the stem connecting the points of 
attachment of the leaves, and these are then marked on 
the spiral, it is found that in any particular species there is 
always a certain definite number of leaves on any given 
nimber of circuits made by the spiral round the stem. The 
-. course of the spiral from any one leaf to the next leaf which 
stands exactly vertically above or beneath it is therefore 
termed the /eafcycle. In order to determine the law of 
phyllotaxis or leaf-arrangement in any particular species, the 
number of leaves in a cycle is placed as the denominator 
of a fraction, and the number of circuits round the stem in- 
cluded in the cycle as the numerator. Thus, for example, 
the es of the pear, oak (Fig. 140), poplar, and walnut, 
have a 2 arrangement ; zz. five leaves are found on every 
two circuits which make up the cycle. In Plantago (Fig. 
141) the phyllotaxis is 8. That the law of phyllotaxis, to 
which the floral leaves are also subject, must have a great 
influence on the estivation is self-evident: thus the quin- 
