370 EQUISETALES 
organism, as is clearly seen in the case of such species as Z. pratense 
(Fig. 192). In other species again the development or non-development 
of the branches differentiates the vegetative axes from those which are 
fertile, as in EZ. arvense and maximum: in others the lateral branches 
on fertile axes are only delayed in their development, as in Z. palustre 
and sylvaticum : in others again there is little difference as regards branching 
between the fertile and sterile shoots. But it has been shown experimentally 
by Goebel! that even in so pronounced a case of the absence of lateral 
branches as the fertile axis of &. arvvense the development of green lateral 
branches could be induced: this was done by culture of the lower 
internodes in a moist chamber, when green assimilating branches were 
put out from the nodes, as in the vegetative shoot. The apparently 
branchless fertile shoot was thus brought into line with the ordinary 
branched type prevalent in the genus. Such facts indicate that the 
branched condition was probably common for the genus, but in certain 
cases a late differentiation has arisen between the colourless fertile shoots 
where the branches are dormant, and the branched green assimilating 
shoots.? 
The fertile strobilus of Zgu¢se¢wm is normally terminal on the axis, 
and is usually borne on the relative main axis only. Many cases exist, 
however, of the development of the strobili on lateral branches: this may 
be normal for certain species, such as &. myriochaetum, Cham, of the 
sub-section Fletostachya, Milde, well shown in Engler and Prantl, Péanzen- 
Jamilien, i. 4, Fig. 343, p. 54753 but it also occurs occasionally in others, 
where a single terminal strobilus is normally present (“forma polystachya”).§ 
In the case of Lgutsetum sylvaticum polystachyum (Fig. 194), where 
numerous lateral branches normally sterile bear small strobili, Luerssen 
has been able to correlate the change with external conditions:* this is 
the next step to bringipg its determination within the limits of experiment. 
On the other hand, numerous cases have been recorded of the continued 
growth of the strobilus, at its apex, with a return to the ordinary vegetative 
characters. Such facts show that the lateral brariches are not essentially 
different from the relative main axis, as regards the final end of spore- 
production : also, that there is no absolute barrier between the vegetative 
and the fertile regions in Zgudsetum. Speaking generally, the fertile strobilus 
is not restricted to axes of any definite order. Thus it requires no great 
effort of imagination to see in the shoot-system of Zyudsefum the result 
of amplification of a simple unit, the shoot, composed of axis and successive 
1 Ber. d. Deutsch. Bot. Gesell., 1886, p. 184. 
* For an interesting discussion of the biological relations of the sterile and fertile shoots 
in living species of Zyzdsetum, see Goebel, Organography, vol. ii., p. 501. 
*¥or records of such developments in European species, see Rab. Avypt. Flora, iii., 
p- 622, etc.; and especially Luerssen, ‘‘ Beitr. z. Kenntn. d. Flora, W. and Ostpreussens,” 
Bibl. Bot., 1894, Heft 28. 
4 Zee, pe 13. 
