23O DIFFERENTIATION OF TISSUES. FUCUS 



two views deduce their shapes. Draw under the high power 

 samples of the cells which are not identical in appearance in 

 transverse and longitudinal sections. Note the thin transverse 

 walls of the conducting cells. 



(5) Examine transverse and longitudinal sections of the stipe, 

 noting the differences between its structure and that of the frond, 

 especially (a) the worn surface with radial splits between the 

 surface cells ; (b) the absence of a distinct palisade layer and 

 the radial rows of cortical cells with relatively thin tangential 

 walls, indicating that this layer has been largely formed by 

 secondary cell division parallel to the surface ; (c) the mass of 

 fibres (" hyphae ") mostly cut transversely, forming the medulla, 

 interspersed with large isolated cells (the original medullary cells) . 

 Make drawings to illustrate these points. 



In the longitudinal section of the stipe identify the tissues 

 seen in transverse section. 



(6) Examine a longitudinal section through the apex of the 

 frond, showing the apical cell and the origin of the adult 

 tissues. 



Sexual Organs. 



(7) Examine a section across the reproductive region of the 

 thallus, first under the low power. Note that the vegetative 

 tissues have the same general characters as in the purely vegetative 

 part of the frond, but that the medullary cells here form a net- 

 work, the cells being cut in various directions : this is the result 

 of the increase in thickness of this part of the frond, the strands 

 of cells being drawn out in all, not only in one direction. 



(8) Examine the development and structure of the antheridia 

 and oogonia in the same or in different conceptacles, and draw 

 as many stages of development as you can distinguish. Note 

 also the " sterile " hairs in the conceptacle. 



(9) Examine fresh material in which eggs and sperms have 

 been liberated, and appear as little masses on the outside of the 

 frond. In species with the sexes on separate plants the sperms 

 can be distinguished by their bright orange colour. Mix some 

 eggs and sperms in a drop of sea-water or in salt solution of about 

 the same concentration. Draw under the high power an egg 

 and some sperms and watch the movements of the latter. The 

 early stages of fertilisation (conjugation) can often be seen. 



