494 Agnes Arber .— The Phyllode Theory of 
are quite inconspicuous in comparison with the associated fibres. In the 
cortical bundles of the axis in the Potamogetons, transitions can be traced 
between the rudimentary vascular bundles enclosed in fibrous sheaths and 
mere bast bundles. 1 If the fibrous strands of the leaf of Potamogeton 
zosterifolius are really reduced bundles, it is possible that the leaf originally 
corresponded in anatomy to such a leaf as that of Narcissus pseudo-narcissus 
(Fig. 14, p. 479). If that be the case, the fibrous strands may be regarded 
as a masked and vestigial indication of phyllodic structure. This suggestion 
is, however, highly tentative, especially as there appears to be no means, in 
the case of strands of fibres, for determining the orientation of the pre-existing 
vascular bundle. Whether this suggestion can be extended to other plants 
must, at present, remain doubtful; it is probable that fibrous strands may 
have different origins in different cases. 
(ii) Apical openings. 2 
Openings in the epidermis and subjacent tissues at the apex of the leaf, 
by means of which the tracheides, in some cases, come to be actually 
exposed, occur in many submerged plants. In the case of Dicotyledons, 
these apertures generally arise through the decay and disintegration of 
water stomates or the disarticulation of an apical trichome. But in the case 
of a number of Monocotyledons, the pores arise merely by the general 
destruction of the apical tissues involved, without the loss of any definite 
organ. The present writer wishes to suggest that the tendency to the 
formation of these openings may, in the case of Monocotyledons, be asso¬ 
ciated with the ancestral loss of the lamina and the consequent ‘ unfinished ’ 
condition—if we may so express it—of the apical region of the leaf. The 
tendency of the veins in phyllodes to converge towards the tip might, not 
improbably, lead automatically to the formation of an apical complex of 
tracheides, such as we often find associated with a terminal opening. These 
apical pores are generally regarded as adaptations for maintaining the 
current through the plant by forming a passage for the elimination of water. 
No doubt, in some instances they perform this function, but there are 
other cases, e. g. Hydrocleis nymphoides , Buchen., in which, though the 
apical cavity exists, it remains permanently roofed in with cuticle. 3 It is 
obvious that this arrangement cannot be explained as an adaptation for the 
emission of water, but it is possible that it indicates an inherent state of the 
leaf, which, in other plants, has come to subserve a physiological function. 
1 Raunkiser, C. (1903). 
2 See Sauvageau, C. (1891), Weinrowskv, P. (1899), Minden, M. von (1899), &c. 
3 Sauvageau, C. (1893). 
