The Morphology of Ruppia Maritima. 153 
dermatogen, as he asserts, I was not able to determine. The whole 
group is supposed to represent the vestiges of the primary root 
(Murbeck, 1902, p. 18; Wille, 1883, p. 4), which develops only to 
this rudimentary degree, and never functions. 
The First Adventitious Root. At about the period when the coty- 
ledon and epicotyl have become quite distinct in outline, the form- 
ation of an adventitious root occurs near the base of the epicotyl. 
This grows rapidly, forming a noticeable protuberance (Pl. XIV, 
figs. 112, 113), and in the mature embryo (Text-fig. 28) may be 
seen pointing almost directly upward, or nearly at right angles to 
the position of the cotyledon. 
The distribution of the meristematic regions at the tip of this 
adventitious root is not as clearly marked as in the roots of the 
mature plant. It will be recalled that the roots of the mature plant, 
as in Zannichellia, contain at their tip four distinct meristematic 
regions, representing the initial areas of calyptrogen, dermatogen, 
periblem and plerome. 
In the young adventitious root of the embryo an interesting point 
is the division of the epidermis of the hypocotyl, immediately over 
the young developing root, by 
periclinal walls, apparently to 
form root cap. These divisions 
continue, and anticlinal as well 
as periclinal occur. This isa very 
different condition from that in 
the roots of the mature plant. 
Whether the dermatogen and 
periblem are each also here re- 
presented by a single layer at the 
apex of the growing-point would 
be impossible to assert definitely, 
on account of the irregularity of 
the cells, but such seems to be 
often the case (Text-fig. 29). How- 

Figure 29.— Longitudinal section 
through apex of adventitious root of 
ever, some embryos show only nearly mature embryo; showing p/, 
one layer for both dermatogen plerome, d, dermatogen, and periclinal 
= divisions initiating development of 
and plerome. as Campbell finds calyptrogen, cal. >< 530 
most usual in the primary root 
of Zannichellia (Campbell, 1897, p. 51). The plerome is pretty 
clearly marked (Text-fig. 29), but also does not have as definite a 
point of origin as in the roots of the mature plant. 
