140 A TEXTBOOK OF THEORETICAL BOTANY 



cells, among which the original filaments can be picked out by their thin walls. 

 Towards the base, in the region of the stipe, the medulla contains nothing 

 but these thick hyphae,and the same is true of the flattened edges of the thallus. 



The original medullary filaments are to be seen in the midrib of the thallus, 

 and they run more or less straight vertically, recalling the similar elements 

 in Lamimvia. They are said to act as food-conducting elements and their 

 cross walls are probably perforated, like sieve-plates in the higher plants. 

 Thus we see that Fiicus contains tissues w'hich are specialized for absorption, 

 photosynthesis, conduction and attachment respectively, which implies a 

 remarkably high grade of organization for one of the Thallophyta. 



We must clearly distinguish between the growth in length of the thallus, 

 which takes place only through the apical cell, and increase in the thickness 

 of the branches, which is brought about by growth and division of existing 

 cells. The cells which make up the medulla are, moreover, not closely 

 packed together, as is true of the tissues of higher plants, but embedded 

 together in mucilage, with the result that these seaweeds are generally 

 extremely slimy to handle and are capable of shrinking or swelling to a 

 surprising extent with variations of the water content. Some species can 

 lose more than 50 per cent, of their weight by drying without being injured. 



In Fiicus there is no asexual method, but reproduction through vegetative 

 propagation may take place by the regeneration of the missing parts from 

 bits of the thallus which are broken oflF from the parent plant. 



Sexual Reproduction 



The reproductive organs develop within the conceptacles. In some 

 species only one kind of reproductive organ develops in any one conceptacle, 

 and in such instances, moreover, the sexes are separated on different plants. 

 This varies in difl^erent species ; in some, e.g., F. serratus, the two kinds 

 of sex organs are found on different plants ; in others, e.g., F. spiralis, both 

 types of sex organs may occur in the same conceptacle. The gametes are of 

 two quite distinct types, that is to say, the plants show an advanced condition 

 of oogamy (Figs. 128 and 129). 



The conceptacles arise from superficial cells very near the growang 

 point. A cell becomes depressed below the surface and then divides trans- 

 versely. The upper cell elongates like a tongue towards the opening of 

 the depression and degenerates early, while the lower cell divides vertically 

 to form a group of cells which become the lining layer of the mature con- 

 ceptacle. In some other members of the Fucales, e.g., Himanthalia, the upper 

 cell forms a typical multicellular hair, while in Pelvetia it forms a group of 

 hairs. The suggestion of a connection with the cryptostomata or sterile hair- 

 pits is obvious. 



The sex organs consist of oogonia and antheridia. The oogonium arises 

 from a superficial cell of the conceptacle, which divides to give a basal cell 

 and an oogonial cell (Fig. 130). The basal cell divides no more, but the 

 oogonium enlarges and its nucleus divides into eight, with reduction of the 



