220 THE SECOND BOOK OF BOTANY. 



fibrous cells of wood. In leather-wood the bast- 

 cells are even longer than those of basswood, being 

 from % to -J of an inch long, and ^^ o f an i ncn j n 

 diameter, while those of the wood are not more 

 than -j-J-g- of an inch long. Few fibres, however, 

 are as long as those of leather-wood. There are 

 very few plants in which they exceed -% of an inch 

 in length. 



EXPERIMENT. Strip the bark from various woody 

 twigs, and find the liber. Observe the differences it 

 presents in different kinds of wood. Outside the 

 liber no woody tissue is found, but in very young 

 woody stems this external layer consists of loose, 

 green, cellular tissue. As growth proceeds, this is 

 soon covered with a brown layer of varying hue and 

 thickness, called the corky envelop. The cellular 

 layer, thus covered in, is known as the green or 

 cellular layer. The corky envelop and green layer, 

 taken together, are called suber. 



EXPERIMENT. Gather the bark of as many differ- 

 ent kinds of trees as you can. Separate the suber 

 from the liber. Find the green layer and the corky 

 layer, and note the differences presented by your 

 collection. You will thus associate in your mind 

 the character of the bark with what you know of 

 the other parts and characters of each kind ex- 

 amined. Passing from the centre out, we have 

 Pith ; Medullary sheath ; Layer of wood ; Medullary 

 rays ; Cambium ; Liber ; Suber, composed of the green 

 layer and corky envelop. 



What parts of the stem are cellular ? What parts 

 are fibrous and vascular? Do you know which 

 threads are warp and which are woof, in cloth ? If 



