64 



MORPHOLOGY OP SPERMATOPHYTES 



contour occupies the center. The longitudinal rows of cells 

 which compose this converge at the rounded apex toward a small 



initial group of cells. The ple- 

 rome is surrounded by a mantle 

 of periblem consisting of many 

 (in Thuja occidentalis 12 to 14) 

 concentric layers arranged with 

 considerable regularity. Each 

 one of the inner layers (in 

 Thuja 8 to 10) of this mantle 

 has its initial group above the 

 apex of the plerome." Cell di- 

 visions perpendicular and paral- 

 lel to the surface increase the 

 surface elements of the layer 

 and the niimber of layers. " As 

 the layers are pushed outward 

 above the apex by their succes- 

 sive doubling, division ceases in 

 them, and increase of volume of 

 the cells takes place ; those which 

 happen to be outermost at the 

 apex become gradually loos- 

 ened, and pushed off as a root 

 cap. Here, then, it is not possi- 

 ble to distinguish a layer of calyptrogen or of dermatogen; the 

 outermost periblem acts as root cap covering the meristematie 

 apex." 



The vascular axis is mostly diarch, a simpler structure than 

 in Dicotyledons generally, and much simpler than in Monocoty- 

 ledons. 



Fig. 48. — Juniperus oxycedrus, longi- 

 tudinal section of lateral root : f-p^ 

 plerome, surrounded by about sixteen 

 layers of periblem, the outermost of 

 which represent the root cap ; i, ini- 

 tial region for periblem and plerome. 

 — After Db Babt. 



II. THE SPORE-PRODUCING MEMBERS 



THE MICEOSPOBANGIUM 



The strobili of Conifers are ahvays monosporangiate, either 

 monoecious, as in Abieteae and Thuja, or dioecious, as in Arau- 

 caria, Juniperus, and Tax us. The microsporangiate strobili 

 are usually much more numerous than the megasporangiate 

 ones, and are never terminal on the primary axis, or even upon 



