THALLOPHYTA— ALGiE 43 



The sub-divisions of the group are very difficult to define, 

 and many schemes of classification have been suggested. 



Several different types may with advantage be considered. 

 Of these the Fucace^ are most familiar. The thallus is 

 generally of large size and shows a copious branching. In 

 Cystoseira and other genera the shoot can be seen to be divided 

 into stem and leaves. SargassiLtn exhibits also the fruiting 

 branches spoken of above. The branches of the thalloid shoot, 

 or the leaves of the leafy forms, often exhibit curious air- 

 chambers which serve as floats. The plants are often attached 

 to the substratum by suckers, or by haptera. The epiphytic 

 forms are secured to their hosts by haustoria, which penetrate 

 the tissues of the latter. 



The thallus generally shows histological differentiation. 

 There is an external rind of closely packed cells, in which the 

 colouring matter is abundant. This constitutes a pseudo- 

 epidermis. The innermost layer of this tissue is capable of 

 increasing the thickness of the shoot by repeated tangential 

 divisions, forming in some cases almost a rudimentary bark. 

 Beneath this layer is some thick-walled parenchyma ; and this 

 covers in turn a central strand, often of considerable dimensions. 

 The cells of this strand are elongated and narrow, and have 

 their longitudinal walls pitted. They do not, however, become 

 woody, but their walls are usually mucilaginous and much 

 swollen. 



Growth in length is carried out by means of an apical cell, 

 which is placed at the bottom of a pit or depression at the apex 

 {fig. 792). The branching may be lateral or dichotomous. 



The reproductive organs are variously situated, but always 

 occur in peculiar depressions of the surface known as con- 

 ceptacles {fig. 793 t). These are nearly globular cavities in the 

 thallus which open to the surface by small pores or apertures 

 known as ostioles. They are developed near the growing point. 

 One or more cells cease to grow, and by the continued increase 

 in size and number of those adjoining them a pit is formed 

 which becomes ultimately the cavity of the conceptacle. The 

 distribution of the sexual organs in the conceptacles varies in 

 different species. In some, antheridia or oogonia arise in each ; 

 in others the same conceptacle may contain both. 



The cells at the bottom of the conceptacle grow out into hairs, 

 from some of which the sexual organs arise. The antheridia 

 are formed in great numbers as the terminal cells of branches 

 of these hairs {fig. 795). Each produces a number of bi-ciliated 



