192 STRUCTURAL BOTANY 



basal disc and lower parts of the thallus are gener- 

 ally thick-walled. Their function is to strengthen the 

 plant mechanically, and it will be found that the lower 

 portion of the thallus, where these hyphae are most 

 abundant, offers the greatest resistance to tearing. 



Although it is thus possible to distinguish several 

 systems of tissue in mature parts of the thallus, it 

 must not be supposed that the different layers remain 

 permanently distinct one from another. As a matter 

 of fact the cells of one system constantly give rise to 

 those of another. For example, the outermost assimi- 

 lating cells divide tangentially, and the inner daughter- 

 cells, thus cut off, contribute to the more internal cortical 

 parenchyma, which appears to discharge the function 

 of storing the assimilated food. Again, the cells of 

 the inner cortex grow in length and may give rise to 

 hyphae, thus adding to the bulk of the central tissue. 

 The elements which correspond to sieve-tubes may 

 subsequently undergo further elongation, thicken their 

 walls, and assume the part of mechanical elements. 



In the older parts of the thallus the assimilating 

 layer dies away, and is replaced by a secondary tissue 

 answering the same purpose, produced by the repeated 

 divisions of the underlying cortical cells. Thus we 

 see that the various kinds of tissue, which appear so 

 distinct when fully developed, can be derived the one 

 from the other. Owing to the cell-formation in the 

 superficial and other layers, and to the growth of new 

 hyphae which insinuate themselves among the old, a 

 constant increase in the thickness of the thallus goes on. 



True starch is not formed as the result of assimila- 

 tion ; it is represented by another carbohydrate occurring 

 in granules, but not capable of being stained by iodine. 



