108 A. H. Graves, 
noted that the axillary scales probably serve a similar purpose of 
protection for the young growing points of the stem. 
The references in literature to a similar root sheath or coleorrhiza 
are scanty. As stated above, Irmisch gives it a mere mention in 
his work, as occurring in Potamogeton, Ruppia and Zannichellia. 
A similar formation in Lemna is described by Caldwell (1899). 
Long ago Strasburger (1873) noted and described an epidermal root 
sheath in Azolla. According to him the sheath was formed of two 
layers of cells, the inner layer eventually disorganizing and leaving 
the root free inside the sheath, which itself surrounded the root 
during its entire existence. Leavitt (1902), however, finds only one 
layer producing the sheath in Azolla, and through this the root soon 
pierces, leaving it as a collar at its base. Quite recently Lachmann 
(1906) has described an essentially similar structure in Ceratopteris 
thalictroides.* 
2. The Growing Point of the Root. 
Four meristematic layers appear well defined at the root tip 
(Pl. VI, fig. 31; Pl. VIII, fig. 44). The calyptrogen is a well marked 
region with several rows of tabular cells. The dermatogen layer, 
one cell thick, encloses all of the remainder of the meristematic tissue 
of the root. This dermatogen layer is continous, even at the apex. 
Inside of the dermatogen layer the periblem rows converge until 
they meet at the apex in a small cell area one or often two cells 
deep. At this same area the plerome originates, and the sides of 
the plerome cylinder are generally clearly marked off from the 
surrounding periblem. 
Miss Daisy G. Scott (1906) in a recent paper has published the 
results of her investigations of the root tips of several aquatic 
monocotyledons. Among these are Ruppia, Naias and Zostera, in 
all of which the periblem and dermatogen arise from a single initial 
cell. These contain, then, according to Miss Scott, three apical 
meristematic regions, giving rise, the first to calyptrogen, the second 
to dermatogen and periblem together, and the last to plerome. 
As will readily be seen, this arrangement disagrees with what I 
have described for Ruppia. Four meristematic regions are always 
clearly defined here. The dermatogen is invariably a continuous 
single layer, and the periblem arises quite often from a two-layered 
region at the apex, always showing at least one layer (PI. VIII, 
fig. 44), 
1 Cf. also Goebel, 1898, pp. 469—470. 

