GENERAL MORPHOLOGY 369 
markings of the leaf-sheath next above: at the nodes it is clearly seen that 
the teeth of the successive leaf-sheaths alternate. The leaves themselves 
are mostly dry and chaffy, while the tissues of the stem contain chloro- 
phyll, and constitute the chief assimilating tissue of the plant. The 
number of teeth in the sheath, their proportions, and their permanence 
or deciduous character may vary: the internodes may be swollen for 
storage purposes in underground stems, while on the aerial stems the 
extent of the chlorophyll-parenchyma, and the number and _ disposition 
of the stomata may fluctuate; but, putting aside such differences, which 
are only of secondary importance, the plan of the shoot is the same 
in all living Horsetails. It is a notable fact that in none of them is there 
any departure from the radial symmetry of construction of the shoot, or 
from the verticillate disposition of the leaves. 
The normal branching of the shoot is exclusively monopodial,! and 
originates from cells lying immediately above the leaf-sheaths, and in a 
position alternating with its teeth (cells marked 4, 4 in Fig. 193); the 
branches are therefore not axillary. The shoots thus initiated burst 
through the subtending sheath, giving the appearance of an endogenous 
origin, and on further development they repeat, though usually on a 
simplified scale, the characters of the original shoot. Such branches are 
not initiated at every available point intervening between the leaf-teeth: 
moreover, where they are initiated, they are frequently not developed 
beyond’ the earliest stages, in which case there may be no external sign 
of their presence. The branches thus formed are plainly accessory to the 
parent shoot, while they repeat its characters: they are not to be held 
as any necessary constituent part of the parent shoot, but as parts added 
to those of the simple shoot itself. 
The roots, excepting the primary root of the embryo, are formed in 
regular relation to the accessory buds above described: one root is 
initiated at the base of each bud, and thus the roots, though formed 
like. the buds in definite positions relative to the other parts, are 
held none the less to be accessory also. Their further branching is 
monopodial. 
Both roots and shoots are susceptible of different degrees of development 
according to circumstances, with results which lead to striking external 
' differences; and upon these the specific distinctions are partly based. 
Either shoots or roots may remain dormant though initiated: this is 
especially seen in the case of the roots in aerial parts, and of the lateral 
.shoots in the parts that are underground. This circumstance provides 
specific characters: thus, in some species many or all of the branches 
may remain dormant, even on the aerial stems (eg. £. dmosum and 
hiemale). It also contributes largely to the general aspect of the individual 
1Qccasional terminal branchings have been described, especially in the region of the 
strobilus, which would be comparable with the terminal branchings in the Lycopodiales, 
but they are sufficiently uncommon to be held as abnormalities. 
2A 
