PTERIS 11HIZOME 299 



thick, brown, stratified walls : the cells contain a somewhat re- 

 duced protoplasmic body. 



Examine also the parenchymatous ground-tissue, which will 

 be found to consist of cells with thin cellulose walls, and mu- 

 cilaginous protoplasmic contents, with much starch. Note 

 especially small pegs and rods, which appear as superficial out- 

 growths from the walls which adjoin the intercellular spaces. 



Select a single elliptical vascular bundle for detailed observation 

 under a high power, and recognize 



1. The bundle-sheath or endodermis, which is a definite, but 

 narrow layer of cells, with brown coloured contents. 



2. The phloem-sheath or pericycle, which is not quite so 

 definite a layer of deeper cells, with cellulose walls, and plentiful 

 protoplasm. 



3. Within this is a band of phloem, which is wider at the 

 flattened sides of the bundle, but narrower at the two poles : it 

 consists of 



a. Sieve-tubes, which may be recognized by their large cavity, 

 sparing contents, and cellulose walls : note where two sieve-tubes 

 are contiguous that bright yellow granules may be seen adhering 

 to the wall : these indicate the position of the sieve-plates. 



b. Parenchymatous cells, which fall under the general term of 

 conjunctive parenchyma. 



4. An elliptical area of xylem occupies the centre of the 

 bundle, it is composed of 



a. Tracheides, which are large and of circular or polygonal 

 outline, with highly refractive lignified walls, and no cell-contents. 

 Towards the poles of the elliptical area may be seen tracheides 

 of small size, and circular outline : these are the protoxylem. 



6. Conjunctive parenchyma, distributed irregularly among 

 the tracheides. 



Longitudinal sections should also be cut, so as to traverse the 

 vascular bundles of the central series : they may be treated as 

 above directed. Note in these the thin-walled ground-paren- 

 chyma, and the brown sclerenchyma ; the latter consists of 

 fibrous cells with peculiar crossed pits in their walls. 



a. The bundle-sheath, or endodermis, consists of narrow 

 oblong cells with square ends, and with brown contents. 



