216 
MORPHOLOGY OF MEMBERS. 
clothed with the remains of the leaves, or are naked, as in Tree-Ferns, procumbent 
species of Aspidium^ Palms, species oi Aloe, &c. 
If a comparison is made between the amount of development in bulk which 
takes place in the leaves and in the axis of a shoot, we find as extremes on one 
side, for example, the Cacti {Cereus, Mammillaria, Echinocactus, &c.) with gigantic 
axes and entirely abortive leaves, on the other side the Crassulacese with fleshy 
crowded leaves and comparatively weak stems; or on one side the underground 
tubers of the Potato with scarcely visible scales, and on the other side the bulbs 
of Liliaceae with fleshy scales which entirely envelope the short stem. 
In reference to the formation of leaves which appear on the shoots, it must 
first be noted whether the same axis always produces only similar leaves or such 
as gradually vary in form. The first is the case, for example, in most MUscineae, 
Ferns, Lycopodiaceas, Rhizocarps, all Equisetacese, and most Conifers; the latter, 
on the other hand, occurs commonly in shrubby Dicotyledons. In Monocotyledons 
and Dicotyledons (to a certain extent even in Conifers) it not unfrequently happens 
basal parts of the leaf-stalks ; 7 a young leaf; b a decayed leaf-stalk, the basal portion of which is still living and bears a bud 
Ilia; the hairy threads are roots which arise behind the growing apex of the stem. 
that the different forms of leaves are distributed over different generations of shoots ; 
certain shoots produce, for example, little or nothing but foliage-leaves, others 
produce only bracts with or without flowers {e. g. Begonia). In such cases the 
shoots may be designated, according to their leaves, scaly shoots, leafy shoots, 
l^ract-axes, flowers, peduncles, &c. On this point further details will be given in 
Book II. 
It is of very common occurrence with Cryptogams and Angiosperms (not 
with Gymnosperms) for a persistent primary axis or branch-system to continue to 
grow underground, and to send up only at intervals long foliage-leaves or shoots, 
which subsequently disappear in their turn and are replaced by others. When 
such axes or branch-systems lie horizontally or obliquely in the ground, and 
produce lateral roots, they are called rhizomes (Fig. 156), (as in Iris, Polygonaium, 
Pieris aquilina and many other Ferns). Frequently they die at the posterior and 
continue to grow at the anterior end. Underground tubers and hulls are more 
transitory structures, usually lasting only for one period of vegetation ; the former 
