Lecture XXV. 229 



less impermeable. Passage cells, the walls of which are not cutinised, are 

 seen at intervals in the endodermis. Within the endodermis is the conducting 

 tract. Its outermost layer is the pericycle, composed of email thin-walled cells 

 retaining their protoplasmic contents. The vessels and tracheids of the root 

 occupy the centre of the cross section and appear in it as a stellate patch. The 

 primary wood forms the central part ef this patch while the spiral tracheids 

 of the protoxylem form the tips of the rays of the star. There are usually 

 four such rays of protoxylem. They are radially opposite the passage cells 

 of the endodermis. The bast appears as narrow thin-walled elements be- 

 tween these rays. 



The transverse section of the stem appears as a perforated disc, the greater 

 part of which is composed of fundamental tissue. It is bounded on the out- 

 side by the epidermis. The majority of its cells are almost square in trans- 

 verse section. Their outer walls are thickened and cutinised. Occasional 

 tapering hairs are seen which are out-growths of single epidermal cells. 

 Unlike the root hairs their walls are thick. Within the epidermis are the 

 cortex, medullary rays and pith. The characteristics of the cells composing 

 these tissues have already been described. The structure of the vascular 

 bundles and their sheath is easily made out. Careful drawings should be 

 made of the whole cross section under the low power showing the position 

 of the bundles and of parts of the various tissues under the high power. 



Drawings should also be made of the transverse section of the leaf, show- 

 ing the characteristics of the epidermis, palisade and spongy parenchyma. 



In addition to these sections, sections should be made of the stem of the 

 plant whose roots have been immersed in the eosin solution. They should 

 be cut with a dry razor and transferred to a dry slide. Without covering 

 them they should be examined with a low power. It will be seen that the 

 eosin, which has entered the roots, has been drawn up in the wood and 

 occupies the cavities of the tubes of the wood. It stains their walls, and in 

 time spreads to the adjacent tissues. The observation shows that the tubes 

 are the water carriers in the tract. 



