io6 Practical Plant Biology. 



medulla. Like the cell-walls of the inner cortex, those of the 

 medullary cells are thick. Where the cells come into contact 

 with one another the walls have thin spots (pits) upon them and 

 these thin spots are finely perforated. Through these perforations 

 excessively fine strands of protoplasm pass, linking up the proto- 

 plasm of adjacent cells. Similar connections are found between 

 the cells of the cortex. The cells of the medulla possess very few 

 chromatophores but they are peculiarly rich in tannin granules. 

 Evidently the tissue of the medulla acts as a storehouse for the 

 latter material. 



In the mid-rib the medullary hyphae are disposed parallel to one 

 another and run longitudinally in the frond. Their elongated 

 form, added to the fact that they possess terminal perforations in 

 their walls, fits them probably for transmitting substances and for 

 conveying stimuli in a longitudinal direction. 



Where the transverse section passes through one of the tufts 

 of hairs emerging from the surface, it will be seen that the hairs 

 take their origin from the sides of a small depression in the 

 surface of the frond and pass out through a small orifice. The 

 margin of the orifice is often slightly raised above the surrounding 

 tissue. The hairs expose a large surface to the surrounding 

 water and probably function as absorbing organs for the mineral 

 salts needed by Fucus in its nutrition. 



Observation shows that Fucus grows by additions made to the 

 ends of the branches. On the very edge or tip of a growing branch 

 there is a small groove running in the same plane as the frond is 

 extended. Microscopic investigation shows that the bottom of 

 this groove is formed of a linear series of large rectangular cells 

 which frequently divide by means of partition walls. These walls 

 cut off segments parallel to the inner and longitudinal sides or 

 faces of the cells. The segments formed on the longitudinal 

 faces subdivide and give rise to the cells of the cortex while 

 those cut off from the inner surface develop into the medullary 

 hyphae. Thus from the growing groove all the tissues of the plant 

 are formed. 



Sometimes the middle cells of the series forming the bottom of 

 the groove cease dividing while those towards either end continue. 

 In this way two groups are formed from the originally continuous 

 series and as each continues growth and subdivision two growing 

 regions are developed, each of which ultimately gives rise to a 

 branch, and bifurcation, or dichotomy, results. 



Normally the central cells of the groove give rise to the mid-rib 

 composed of its elongated medullary hyphae and the cortex, while 



