THE THALLUS OF THE HIGHER ALG^S. 2/ 



youngest region, found also at the tips of the growing axes of 

 the higher plants, is the growing point. It has no limit below, 

 but as the parts further and further from the apex are exam- 

 ined, they are seen to become more and more unlike with age 

 until the mature form is reached. 



36. Mature thallus. — If the mature part of the thallus be 

 cut at right angles and a thin slice be cut off one end, placed 

 on a glass slip and examined with a lens, it shows two distinct 

 regions ; a central one, quite translucent, the pith, bounded 

 by an outer brownish opaque region, the cortex. The 

 central part is very full of mucilage, produced by a change 

 in the substance of the cell- walls of the pith region. In the 

 bladders this mucilaginous pith does not increase to fill the 

 central space, but this is occupied by a great chamber filled 

 with air and other gases. In the midrib the structure is 

 plainly denser than elsewhere, except in the stalk below, 

 which is like an enlarged midrib without the side wings. 



37. Division of labor. — Complete examination of other 

 parts, the attachment disk, the hair pits (fig. 28) with which 



Fig. 28. — A transverse section of the thallus of Fucus, showing midrib, r ; cortex, c ; 

 pith, m ; and a hair pit, /*. Magnified 10 diam.— From a drawing by Mr. C. E. Allen. 



many species are covered, etc., would reveal still other ways 

 in which unlikeness arises with age from the uniformity of the 

 growing point. With the change of form there is always di- 

 vision of labor, which we can interpret only in a very imper- 

 fect fashion from our own standpoint. The corn-pact cortex 

 is nutritive and probably in part protective ; the bladders serve 

 to increase the buoyancy of the plants when the tide is in ; 

 while the abundant mucilage, found in the interior, probably 

 serves to retain the moisture when the plants are exposed by 



