G YMNOSPERMJE. 



163 



gives rise, by the formation of a transverse partition, to a 

 cell which is the rudiment of the suspensor. This grows 

 and elongates, and at its lower end the embryo is formed ; 

 the growing point {pundum vegetationis) is opposite to the 

 suspensor ; and near it, as lateral members, are produced 

 the cotyledons. At the end next to and under the sus- 

 pensor, a root, with a few-celled root-cap, is formed. The 

 epidermal system consists of one or more layers of epi- 

 dermal cells, frequently much thickened. The xylem 

 portions of their fibro-vascular bundles are compacted into 

 a dense woody cylinder, surrounded by the so-called bark 

 or united phloem portions of the bundles. The generating 

 tissue, or meristem, called 

 cambium, is situated between 

 the phloem (bast) and xy- 

 lem (wood). The mass of 

 xylem is formed of trachei- 

 des, with thickened walls and 

 bordered pits (Fig. 278). The fundamental tissue consists 

 of parenchyma in the inner portion (pith), which soon loses 

 its vitality ; the outer portion (cortex) consists of paren- 

 chyma and sclerenchyma, or collenchyma; in it there is 

 considerable development of cork. The narrow radiating 

 plates of tissue (medullary rays) between the fibro-vascular 

 bundles, are parenchymous in the young, and sclerenchy- 

 mous in the older, stems. The medullary rays have cam- 

 bium, called interfascicular cambium, corresponding in 

 position and function with that of the fibro-vascular 

 bundles. Most Gymnosperms have intercellular canals, 

 filled with turpentine, containing dissolved resin. The 

 class is divided into three orders, as follows: Cycads 

 {CycadecB), Conifers (^Coniferce), and Joint Firs (Gnetacece). 



Fig. 273. Bordered Pits, and diagrams reprcsenling llieir development. 



278 



