FUCALES (HETEROGENERATAE) 193 



not been studied in much detail but the following figures (Table I) 

 may be cited from one worker who marked a number of plants : 



Table I 



Ascophyllum 

 Species ... F. spiralis F. serratus F. vesicHlosus nodosum 



Max. age (yr.) 3^ 4 2^ 2^ 



Av. age (yr.) i^ 2 i i| 



Morphologically the thallus shows considerable differentiation. 

 The external layer, which is known as the limiting layer, consists of 

 small cells with abundant plastids and is primarily assimilatory in 

 function. Below this there is a cortex composed of several layers of 

 parenchymatous cells which become more and more elongate and 

 mucilaginous towards the centre, and these probably form the 

 storage system. In the very centre the cells are extended into 

 hyphae which are interwoven into a loose tangled web. This central 

 tissue is called the medulla and probably acts as a conducting 

 system, because the transverse walls of the hyphae are frequently 

 perforated with the same type of pit that is to be found in some of 

 the Laminariaceae. The primary medullary hyphae are relatively 

 thin- walled, but when secondary growth of the thallus takes place 

 the new hyphae which result from this process are very thick- 

 walled and so are probably mainly mechanical in function. 

 Secondary growth is due to the activity of the limiting layer and the 

 inner cells of the cortex, the latter tissue being responsible for the 

 formation of the secondary hyphae (cf. fig. 131) which penetrate 

 between the primary medullary hyphae and finally outnumber 

 them. There is a greater development of secondary thickening in 

 the stipe and mid-rib than there is in the frond, whilst in very old 

 parts of the thallus the limiting layer may die off and then the 

 underlying cortical cells take over its function. 



Growth in length takes place by means of an apical cell which 

 lies at the bottom of a slit-like depression that has resulted from the 

 more rapid growth of the surrounding limiting layer. The apical 

 cell is three-sided in young plants whilst in the adult thallus it 

 becomes four-sided, the new segments being cut off successively 

 from the base and four sides, after which they develop into the 

 various tissues (fig. 129). Injury, and also the stimulus provided 

 when the thallus lies on marsh soil, induces new growth in the 



CSA 13 



