Algal cells living in association with the animal.
Some forms, however, such as " Sarcina," have their algal analogues in Palmellaceae among the green algae, while Thaxter's group of Myxobacteriaceae suggests a relationship with the Myxornycetes.
It may be regarded as derived from a wholly dependent sporogonium not unlike that of some of the simpler Bryophyta; the latter are assumed to have arisen from primitive Algal forms, in which, as the first step in the interpolation of the second generation in the life cycle, the fertilized ovum gave rise to a group of swarm spores, each of which developed into a new sexual plant.
(13) Phycomycetes (Algal fungi); (I 4) Phaeophyceae (Brown Algae).
The algal fungi, Phycomycetes, are obviously derived from the Green Algae, while the remaining Fungi, the Eumycetes, appear to have sprung from the same stock as the Rhodophyceae.
This is especially the case in the lichens (symbiotic organisms composed of a fungal mycelium in association with algal cells), which are usually exposed to very severe fluctuations in external conditions.
The fungal part of the organism nearly always consists of a number of the Discomycetes or Pyrenomycetes, while the algal portion is a member of the Schizophyceae (Cyanophyceae or Blue-green Algae) or of the Green Algae; only in a very few cases is the fungus a member of the Basidiomycetes.
The special fungi which take part in the association are, with rare exceptions, not found growing separately, while the algal forms are constantly found free.
The view of the dual nature of lichens had hitherto been based on analysis; the final proof of this view was now supplied by the actual synthesis of a lichen from fungal and algal constituents.
Moller also in 1887 succeeded in growing small lichen-thalli without their algal constituent (gonidia) on nutritive solutions; in the case of Calicium pycnidia were actually produced under these conditions.
The typical heteromerous thallus shows on section a peripheral, thin and therefore transparent, layer, the cortical layer, and centrally a mass of denser tissue the so-called medullary layer, between these two layers is the algal zone or gonidial layer (figs.
Since they are provided with both fungal and algal elements, they are able to develop directly, under suitable conditions, into a new thallus.
Very difficult to settle g, The algal zone (Cystococcus).
More than are of very, irregu500 times.) lar figure, elonc, An isolated mature soredium, with an algal gated, branched or cell (Pleurococcus) in the envelope or hyphae.
Gate flexuose, with only d, Another with several algal cells in optical a proper margin longitudinal section.
In these forms gonidia are found in connexion with the young fruit; such algal cells undergo numerous divisions becoming very small in size and penetrating into the hymenium among the asci and paraphyses.
When the spores germinate the germ-tubes surround the algal cells, which now increase in size and become the normal gonidia of the thallus.
The algal cells are usually controlled in their growth by the hyphae and are prevented from forming zoospores, and in some cases, as already described, the algal cells are killed sooner or later by the fungus.
Theoretically the lichens may be classified on the basis of their algal constituent, on the basis of their fungal constituent, or they may be classified as if they were homogeneous organisms. The first of these systems is impracticable owing to the absence of algal reproductive organs and the similarity of the algal cells (gonidia) in a large number of different forms. The second system is the most obvious one, since the fungus is the dominant partner and produces reproductive organs.
Most systems agree in deriving the major divisions from the characters of the reproductive organs (perithecia, apothecia, or basidiospore bearing fructification), while the characters of the algal cells and those of the thallus generally are used for the minor divisions.
A more important consideration is the occasional absence of this colour in species, or groups of species, with, in other respects, algal affinities.
They undoubtedly represent the lowest grade of algal life, and their distribution rivals that of the Green Algae.
The experiment of Engelmann referred to deserves to be mentioned here, if only in illustration .of the use to which algae have been put in the study of physiological problems. Engelmann observed that certain bacteria were motile only in the presence of oxygen, and that they retained their motility in a microscopic preparation in the neighbourhood of an algal filament when they had come to rest elsewhere on account of the exhaustion of oxygen.
Considering, however, that it is generally believed that Bryophyta and vascular plants are descended from an algal ancestry, it is natural to suppose that, prior to the luxuriant vegetable growths of the Carboniferous period, there must have existed an age of algae.
It may even be regarded as an open question whether some of them may not have arisen independently and represent parallel lines of evolution from Bryophytic or Algal forms. This leads us to consider the question whether any indications exist as to the manner in which the Pteridophyta arose.
Apart from the multitude of supposed fossil Algae described as " Fucoids " but usually not of Algal nature, and never presenting determinable characters, very little remains that can be referred to Palaeozoic Brown Algae.
There can be little doubt of the Algal nature of the fossil, but beyond this it is impossible at present to carry its determination.
The algal cells are never known to form spores while part of the lichen-thallus, but they may do so when separated from it and growing free.
Carruthers, however, in 1872 established its Algal nature, and gave it the more appropriate name of Nematophycus.
No recognizable plant-remains, if we accept one or two doubtful Algal specimens, have so far been yielded by the Cambrian.