y6 THE ALGAE 



These grow as epiphytes and parasites on and in the leaves of 

 various phanerogams. The plants are composed of one or more 

 branched interwoven threads from which vertical filaments arise 

 that bear clusters of stalked sporangia very like those of Trentepoh- 

 lia. Some species bear sterile erect filaments that terminate in hairs, 

 whilst the parasitic species possess rhizoids which penetrate the 

 cells of the host, although it has not been clearly established 

 whether the host cells are killed before or after penetration. Cephal- 

 euros virescens forms the red rust of the tea plant which may cause 

 much economic damage, but the attack is only serious when the tea 

 tree is growing slowly, because during periods of rapid growth the 

 alga is continually being shed by exfoliation of the outer tissues. 

 The disease cannot be controlled by spraying with poisons, but 

 the bushes can be made less susceptible to attack by treating the 

 soil with potash. Another species, C. coffeae, attacks the leaves of 

 coffee plants. The genus is mostly confined to the tropics. 



In recent years four species of this genus, including C. virescens^ 

 have been discovered in New Zealand growing under distinctly 

 warm temperate conditions. 



Trentepohliaceae: Trentepohlia (after J. F. Trentepohl). {Chro- 

 olepus). Fig. 41 

 The species grow as epiphytes or on stones in damp tropical and 

 sub-tropical regions, but they will also grow under temperate con- 

 ditions if there is an adequate supply of moisture. The threads have 

 a characteristic orange-red colouring due to the presence of ^- 

 carotene which is said to be a food reserve accumulated during 

 periods of slow growth, but if this is so it would be expected that 

 it should accumulate under favourable conditions of rapid growth 

 and disappear under unfavourable conditions when growth is slow. 

 This is a feature of its metaboHsm that would seem to require 

 further investigation. The cells contain chloroplasts that are dis- 

 coid or band-shaped and devoid of pyrenoids. Usually both pros- 

 trate and erect threads are present, though the latter are reduced in 

 some species. Growth is apical, and the terminal cells often bear a 

 pectose cap or series of caps which are periodically shed and 

 replaced by new ones. The origin of the cap is not properly under- 

 stood but it is thought to be due to a secretion, whilst its function 

 may be either to reduce transpiration or else to act as a means of 

 protection: alternatively, it may simply be a means of removing 



