174 PHAEOPHYCEAE 



types of lateral connecting branches can be recognized, the con- 

 necting threads and hyphae. The former arise first in the course of 

 development as outgrowths from the individual cells, but even 

 when mature they are composed of only a few cells. The hyphae, 

 which arise later as short branches of small cells cut off from the 

 original vertical cells, can unite with each other or else they grow by 

 cell division until finally they contain numerous cells which sub- 

 sequently elongate very considerably. 



One of the most characteristic features of the genus is the presence 

 in the medulla of "trumpet" hyphae which are modified cells in the 

 connecting threads and hyphae. At a transverse cell wall the ends of 

 both cells swell out to form bulbs, the upper bulb always being larger, 

 but so far no satisfactory explanation of this pecuUarity has been ad- 

 vanced, though it may be due to purely mechanical requirements. 

 The transverse wall is perforated to form a sieve plate and a callus 

 develops on each side, both callus and sieve plate being traversed by 

 protoplasmic strands. It will be seen that in many respects these 

 trumpet hyphae resemble the sieve tubes of the flowering plants, 

 but although the callus is said to be formed in land plants because 

 of changes in^H, so far no evidence has been published to indicate 

 whether this is also true for the Laminariaceae. Apart from the 

 sieve plates the trumpet hyphae also possess spiral thickenings 

 which appear as striations, and here again there is the problem of 

 their interpretation (e.g. are they growth zones?), although it is 

 possible that they have now lost any function they once possessed. 

 The problem of these trumpet hyphae is still subject to consider- 

 able speculation : it has been suggested that they may be a storage 

 or conducting tissue, whilst another suggested function is that of 

 support, but as the plants are commonly submerged the water would 

 seem to fulfil this requirement. In some species many of the other 

 cells also contain pits with a thin membrane across the opening 

 and these presumably facilitate the diffusion of food materials. 



Most of the genera possess systems of anastomosing mucilage 

 ducts which are normally confined to the stipe, although in 

 L. saccharina and L. digitata they enter the fronds as well. When 

 mature there are periodic openings from these ducts to the exterior 

 and their bases are lined with secretory cells. They arise lysigen- 

 ously through an internal splitting of the thallus due to cell 

 disintegration : this is followed by a differential growth so that the 



