=801. Holdfasts.=—These are organs for anchorage which are not true roots. These are especially well developed in some of the algæ (Fucus, Laminaria, etc.). They are usually called _holdfasts_. The holdfasts of the larger algæ are mainly for anchoring the plant. They do not function as absorbing organs, and the structure is different from that of true roots. =802. Haustoria or suckers= is a name applied to another kind of holdfast employed by parasitic plants. In the dodder the haustorium penetrates the tissue of the _host_ (the plant on which the parasite grows), and besides furnishing a means of attachment, it serves as an absorbing organ by means of which the parasite absorbs food from its host. The parasitic fungi like the powdery mildews which grow on the surface of their hosts have simple haustoria which serve both as organs of attachment and absorption, while in the rusts which grow in the interior of their hosts the haustoria are merely absorbing organs. =803. Rootlets, or rhizoids.=—Many of the algæ, liverworts and mosses have slender, hair-like organs of attachment and absorption. These plants do not have true roots. Because of the slender form and small size of these organs, they are called _rhizoids_, or _rootlets_. In form many of them resemble the root hairs of higher plants. CHAPTER XLII. THE FLORAL SHOOT. I. The Parts of the Flower. The portion of the stem on which the flowers are borne is the _flower_ shoot or axis, or taken together with the flowers, it is known as the _Flower Cluster_. =804. The flower.=—The flower is best understood by an examination, first of one of the types known as a “complete” flower, as in the buttercup, the spring-beauty, the blood-root, the apple, the rose, etc. There are two sets of organs or members in the complete flower—(1) the floral envelope; (2) the essential or necessary members or organs. The floral envelope when complete consists of—1st, an outer envelope, the _calyx_, made up of several leaf-like structures (_sepals_), very often possessing chlorophyll, which envelop all the other parts of the flower when in bud; 2d, an inner envelope, the _corolla_, also made up of several leaf-like parts (_petals_), usually bright colored and larger than the sepals. The outer and inner floral envelopes are usually in whorls (though in close spirals in many of the buttercup family, etc.), and for reasons discussed elsewhere (Chapter XXXIV) represent leaves. The essential or necessary members of the flower are also usually in whorls and likewise represent leaves, but only in rare cases is there any suggestion, either in their form or color, of a leaf relationship. These members are in two sets: (1) The outer, or _andrœcium_, consisting of a few or many parts (_stamens_); (2) the inner set, the _gynœcium_, consisting of a few or many parts (_carpels_).