DIV. 



MORPHOLOGY 



77 



but absorb organic substances, has a correspondingly peculiar aspect. 

 It is termed a MYCELIUM, and consists of thin, highly -branched, 

 cylindrical, colourless filaments (Fig. 87 and Fig. 6) called HYPHAE. 

 These penetrate the substratum, such as the humus soil of a wood, in 

 all directions and thus expose a large surface for the absorption of the 



necessary food materials. Parasitic 

 fungi, if not inhabiting the cells, usually 

 send suctorial projections of the hyphae 

 (haustoria) into the living cells of the 

 host plant from the hyphae in the 

 intercellular spaces (Fig. 85). 



7. Division of Labour between 

 the Branches of the Thallus. The 

 most highly-segmented types of thallus 

 are met with in some Siphoneae and 

 in the Brown and Red Seaweeds 

 (PhaeophyceaeandRhodophyceae). The 

 external segmentation of some of these 

 resembles in a remarkable manner the 

 shoot in cormophytes. Some of these 

 Algae attain a great size, the thallus 



FIG. 84. Portion of Cladophora 

 glvnurata. (x 48. After SCHENCK.) 



haust 



FIG. 85. Haustoria (haust) of Peroiiospora parasitica 

 in parenchyma tons cells of Capsdla. hy, The inter- 

 cellular hyphae. ( x 240.) 



of the Brown Alga, Macrocystis, being 45 m. long. A good example 

 of high differentiation is afforded by the Red Seaweed, Delesseria 

 sanguined (Fig. 88), which has leaf-like lateral branches seated on the 

 cylindrical, branched, relatively main axes. In many such forms, 

 besides the formation of attaching organs or haptera and of branches, 

 a further degree of differentiation is attained. Some cylindrical 

 branches continue the growth and branching of the thallus as LONG 

 SHOOTS. Other branches borne on these are SHORT SHOOTS with 



