Order 16. =Laminariaceæ.= The thallus is more or less leathery, and has generally a root-like lower part (Fig. 69) which serves to attach it, and a stalk or stem-like part, terminated by a large leaf-like expansion. Meristematic cells are situated at the base of the leaf, and from these the new leaves are derived. The older leaf thus pushed away by the intercalary formation of the younger ones, soon withers (Fig. 69). Gametes are wanting. Zoosporangia are developed from the lower part of a simple, few-celled sporangiophore, which is an outgrowth from a surface-cell and has a large club-formed apical cell. The sporangia are aggregated into closely packed sori, which cover the lower part of the terminal leaf, or occur on special, smaller, lateral, fertile fronds (_Alaria_). Most of the species belonging to this order live in seas of moderate or cold temperature and occur in the most northern regions that have yet been explored, forming their organs of reproduction during the cold and darkness of the arctic night. _Laminaria_ is destitute of a midrib and has only one terminal leaf. _L. digitata_ has a broad leaf, which, by the violence of the waves, is torn into a number of palmate strips (Fig. 69). _L. saccharina_ has a small, undivided leaf. _Alaria_ has a midrib and special fertile fronds. _A. esculenta_ occurs plentifully on the west coast of Norway and on the shores of Great Britain. _Chorda filum_, a common seaweed, is thick, unbranched, and attains a length of several metres, without any strong demarcation between stalk and leaf. Some attain quite a gigantic size, _e.g. Macrocystis pyrifera_, whose thallus is said sometimes to be more than 300 metres in length. The _Lessonia_-species, like the above, form submarine forests of seaweed on the south and south-west coasts of South America, the Cape, and other localities in the Southern Hemisphere. USES. The large Laminarias, where they occur in great numbers, are, like the Fuci, used for various purposes, for example, in the production of iodine and soda, and as an article of food (_Laminaria saccharina_, _Alaria esculenta_, etc.). _Laminaria saccharina_ contains a large quantity of sugar (mannit) and is in some districts used in the preparation of a kind of syrup; in surgical operations it is employed for the distension of apertures and passages, as for instance the ear-passage. It is by reason of the anatomical peculiarities and structure of the cell-walls, that they are employed for this purpose. The cell-walls are divided into two layers, an inner one which has very little power of swelling, and an outer one, well developed and almost gelatinous--the so-called “intercellular substance”--which shrivels up when dried, but can absorb water and swell to about five times its size. The stalks of _Laminaria clustoni_ are officinal.