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 

 o'f 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 shoof, or 

 from the verticillate disposition of the leaves. 



The normal branching of the shoot is exclusively monopodial, 1 and 

 originates from cells lying immediately above the leaf sheaths, and in a 

 position alternating with its teeth (cells marked /, /, 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 (c.^. E. limosiim and 

 /lie mate}. It also contributes largely to the general aspect of the individual 



Occasional terminal branchings have been desciibed, 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 field as abnormalities. 



2 A 



