366 PRACTICAL BOTANY 



gives rise to numerous unicellular rhizoids, but there are no 

 amphigastria. 



Note especially that the internal cells have their otherwise thin 

 walls strengthened by vertically running brown bars : where cut 

 through, these are seen to be the result of an equal thickening 

 of the walls on both sides. 



Cut also transverse sections, and compare them : these demon- 

 strate 



1. That the thallus which is massive in the middle, thins out 

 at the margins to a single layer of cells. 



2. That the thickening bars form networks in vertical planes, 

 and only occasionally extend in a longitudinal direction. 



V. Remove a number of the rhizoids : mount them 

 in water or glycerine, and examine under a low power : 

 two types of these elongated unbranched hairs may be 

 recognized 



a. Those with smooth walls : these are the more 

 numerous : 



b. Those with peg-like projections of the wall into 

 the cavity of the hair : these projections are arranged 

 in a more or less clearly spiral manner, and they may 

 not unfrequently be seen to be branched irregularly : 

 the protoplasmic contents are very scanty. 



VI. Avoiding, as before, both the cups with gemmae, and 

 the sexual branches, cut from material hardened in spirit fine 

 median longitudinal sections of the thallus, so as to include the 

 depressed apex : mount in glycerine, and examine under a high 

 power. 



In the older part of the section the structure of the thallus 

 will be seen as above described : on approaching the apex, note 

 that the air-cavities are successively smaller, and the chlorophyll- 

 containing tissue gradually disappears. The thallus has a blunt 

 apex covered by numerous amphigastria, which thus protect it. 

 At the extreme organic apex may be seen a single wedge-shaped 



