LINUM USITATISSIMUM 381 



two cells becomes the hypophysis and the lower one contributes 

 to the development of the suspensor. In the case of the six-layered 

 octant, derivatives of the cell /' form the hypocotyl; and the upper 

 daughter cell, d, derived from the cell m, produces the hypophysis 

 after one or two subsequent transverse segmentations while the 

 lower daughter cell, /, enters into the structure of the suspensor. 

 In the least frequent case, in which the octant consists of five 

 layers (Fig. 193, 13), the layer /' produces the hypocotyl and the 

 layer m, the hypophysis. This divides transversely to form two 

 cells which enter into the development of the two outer histogens 

 of the growing point of the root tip. The upper cell contributes 

 initials to the periblem, while the lower one becomes a part of 

 the calyptrogen-dermatogen layer that produces the root cap 

 terminally and the epidermis laterally. (Fig. 193, 19-30.) 



The Primary Root. — The primary root has a diarch protostele 

 with two rather large groups of primary phloem flanking the 

 primary xylem strand. (Fig. 197, A.') Tognini (15) has reported a 

 special case in which seedlings differentiated three cotyledons, 

 and the six downwardly diverging cotyledonary traces were united 

 in pairs in the lower hypocotyl so that the primary root had a 

 triarch stele. 



There are three to five elements in each protoxylem group, and 

 the metaxylem vessels are successively larger toward the center. 

 The pericycle and endodermis are single-layered and the Casparian 

 strips of the latter are laid down on the radial and end walls before 

 the metaxylem is mature. The cortical cells are isodiametric with 

 large intercellular spaces, except for the compact hypodermal layer 

 in which the cells are somewhat radially elongated and resemble 

 the epidermal cells. The cortex persists for some time and when 

 disintegration occurs, as a result of secondary thickening, it begins 

 in a midregion between endodermis and hypodermis. 



The ontogeny of the primary root has been investigated by 

 Janczewski (13), who used it to illustrate a special variation in his 

 third type of angiospermous roots in which the plerome and peri- 

 blem are sharply defined, and are covered by a common initial 

 layer which gives rise to the epidermis and root cap. In most 

 roots with this type of development, the periblem consists of a 

 single layer at its apical point and becomes wider laterally by sub- 

 sequent periclinal divisions; but, in flax, the apical portion of the 

 periblem consists of two initial layers. In his studies of the devel- 



