112 



A TEXTBOOK OF THEORETICAL BOTANY 



from the organism on which it Hves, and a parasite in which the reverse 

 is the case ; for the parasites hve at the expense of the second plant, which 

 is then termed the host plant. 



The thallus oiEctocarpus is generally filamentous and repeatedly branched 

 (Fig. 92). It is made up of two parts, a creeping portion which covers the 

 substratum and serves as the holdfast, and a number of upright branches 



which arise from it. These upright 

 branches generally consist of a single 

 row of cells, though in some species 

 longitudinal cell divisions occur. The 

 cells are small and rectangular, each con- 

 taining a single nucleus and a number 

 of small spherical bodies containing 

 the brown pigment, which are called 

 chromatophores or chromoplasts. 

 Pyrenoids are present in the chromo- 

 plasts. This is interesting because these 

 bodies, which are characteristic of the 

 Chlorophyceae, are only found in the 

 most primitive members of the Red and 

 Brown Algae. 



Growth takes place by the division of 

 a series of small cells situated near the 

 base of the vertical branches. These 

 cells are generally small and continue 

 throughout the life of the branch to cut 

 off additional cells from their upper sur- 

 faces. The cells of the more apical part 

 of the branch do not divide and are more 

 elongated than those of the actively divid- 

 ing or meristematic region. This is 

 therefore an example of intercalary 

 growth (Fig. 93). 





n 



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i 



Fig. 92. — Ectocarpus sp. Form of 

 plant bearing zoosporangia. 



Asexual Reproduction 



The asexual reproductive organs consist of unilocular zoosporangia 

 which are developed on the side branches (Fig. 94). They originate as simple 

 globular or pear-shaped cells which become densely filled with protoplasm. 

 The original nucleus divides first meiotically and then repeatedly by mitosis. 

 Around each daughter nucleus a small quantity of protoplasm collects to 

 form a zoospore. It consists of a small, rather elongated cell, and contains 

 besides the nucleus a single brown plastid. Two flagella are developed 

 at one side of the zoospore, one pointing forwards and the other backwards. 

 On liberation the zoospore swims away, eventually settles down and grows 

 into a monoploid plant. 



