254 Arber . — Studies on Intrafascicular 
secondary phloem, seem to me to make it impossible to doubt their origin 
from the intrafascicular cambium. From my sections and from the evidence 
of Lignier’s own figure, I thus conclude that the intrafascicular cambium in 
the Arum leaf gives rise not only to phloem, but also to a certain amount of 
secondary xylem. 
In searching for intrafascicular cambium in Monocotyledons, the chief 
difficulty arises in connexion with those families that are mainly aquatic, 
and in which the vascular skeleton is correspondingly reduced. The poor 
development of the bundles in many of the representatives of such families 
probably accounts for the fact that the existing records of intrafascicular 
cambium in the Helobieae are apparently confined to Andersson’s discovery 
of this tissue in Triglochin , 2 and my note on the existence of ‘ very slight 
and irregular cambial activity ’ in the leaves of two species of Potamogeton . 2 
But I have found, on re-examining the latter genus in greater detail, that 
the cambium need not be described in such qualified terms. Microtome 
sections of apical buds of Potamogeton natans , L., reveal the existence, 
in the foliar bundles, of a well-marked seriation of the elements between the 
protoxylem and protophloem (Fig. 4). In very young stages, the seriation 
may involve only one or two files of cells, but at later stages it becomes 
more extensive. 
There has been no record hitherto of the occurrence of cambium in the 
Alismaceae, Aponogetonaceae, or Hydrocharitaceae, but I have recently 
found that, by cutting microtome series through the shoot apices, and thus 
exposing the vascular bundles in their initial phases, the existence of this 
tissue can be demonstrated in these families also. The petiole of Sagit- 
taria sagittifolia , L. (Alismaceae), represented in Fig. 6, was so young that 
it was only about 0*5 mm. in width, and the lacunae (lac.), that would have 
eventually given a lace-like appearance to the transverse section, were only 
in two or three cases making their appearance. Cambium occurs in the 
median bundle at this stage, but it is inconspicuous, since only one or two 
files of cells extending between the protoxylem (ppx.) and protophloem 
( pph .) are radially arranged. The bundle in this phase very closely recalls 
Queva’s description of the median vascular strand of the very young leaf in 
the Liliaceous genera Uvularia and Tricyrtis . 3 The cambial activity in 
Sagittaria is extremely ephemeral, and one may search in vain for any sign 
of it in older petioles. 
On cutting serial sections through the apical bud of Aponogeton 
distachyum , Thunb. (Aponogetonaceae), I found that there is a distinct 
development of intrafascicular cambium in the rudimentary inflorescence 
axis ; that to which the bundle drawn in Fig. 5 belonged, was so young as 
to be only 0*3 mm. in diameter. 
In the case of the Hydrocharitaceae, I examined serial sections through 
1 Andersson, S. (1888). 2 Arber, A. (1918). 3 Queva, C. (1907). 
