CYCADALES 



83 



difference in size, under the microscope, is so slight, that it might 

 almost escape notice (tig. 73). 



Stems with a vascular cylinder developed from a persistent cam- 

 bium of the familiar dicotyl type are called "monoxylic." When 

 there is more than one zone, each with 

 xylem and phloem, the stem is polyxylic 

 (fig. 74). Cycas, some species of Macro- 

 zamia, and some of Encephalartos are of 

 this type. The zones are formed at ir- 

 regular intervals of, probably, many 

 years, and may mark the number of 

 times the plant has had prolonged dor- 

 mant periods. Where the zones succeed- 

 ing the first stelar zone originate is 

 not settled definitely. Jeffrey^''^ claims 

 they arise in the pericycle, while Mil- 

 LER^^'' claims that pericycle is indistin- 

 guishable from the endodermis and the 

 rest of the cortex. Sister Helen 

 Angela'^'* found cambiums in the cor- 

 tex of Ceratozamia, thus proving that 

 embryonic tissue can arise in the cortex. 

 In some angiosperms, like the beet and 

 some other members of the Chenopo- 

 diaceae, similar zones arise from a well- 

 marked pericycle. In Boerhavia (Nycta- 

 ginaceae) the zones are particularly 

 well marked. The problem is hard to 

 settle because it would kill a valuable 

 plant to get material. 



A stem of Cycas media, 3 meters in 

 height, showed at the base three zones 

 of xylem and phloem. A piece of stem 

 of Cycas pectinala Griff., 20 cm. in diameter, had 14 zones, doubtless 

 a very high number. 



The protoxylem, in seedlings, has spiral markings, but in older 

 plants, where elongation is extremely slow, even the protoxylem 



Fig. 73. — Dioon spinulosum: 

 histology of the growth-rings; 

 the slow growth, corresponding 

 to the summer wood in an or- 

 dinary dicotyl, is at g. — After 

 Chamberlain. •°9 



