During the summer months, Cystoseira osmundacea plants can reach mean heights of around 13 meters, some being measured as long as 80m total length. The processes triggering growing in Cystoseira osmundacea are yet unclear, but many of the intricate mechanisms behind this alga's striking seasonal growth are well understood and paint an exquisite picture of complexity.
Cystoseira displays apical growth, meaning that all new growth develops from the division of one cell at the tip of the branch or blade: the apical cell . In C. osmundacea, the apical cell is three-sided (picture a three-sided pyramid with the top sliced off), which gives rise to the radial arrangement of blades and apical fronds (the apical cell can divide in three different directions). "Radial arrangement" refers to the spiral arrangement of fronds around the stipe. The apical cell is always located in a groove at the tip of the growing frond or blade. This groove, the apical depression , forms a funnel filled with mucilage. At an early stage in development, the apical cell becomes established and will persist throughout the life of the alga.
Apical division also produces the dichotomous and alternate branching of the blades and apical region. The specific type of branching in Cystoseira is called monopodial branching . In monopodial branching, the apical cell divides, and the resulting daughter cell continues to divide, forming a lateral frond or blade. Thus the initial apical cell continues growth along the main axis, while daughter cells form only lateral axes. (This type of branching contrasts with sympodial branching , where the daughter cell forming a lateral axis actually becomes the dominant axis for a short time, until another daughter cell becomes dominant, and so on, producing a "swinging" axis, even if the alga appears to have one main axis.)
In addition to the apical cell, C. osmundacea has other sources of new growth, one of which is the intercalary meristem . This meristematic tissue lies between the stipe and blades, generating new tissue in two directions and elongating the thallus through cellular division.
The meristoderm composes the third area of new growth activity, and its cells contribute to increasing the girth of the thallus. The outer layer of pigmented cells shown to the right are those that contribute to the increased thallus circumference.
Growth in Cystoseira is an oriented process in which the apical and basal polarities are established very early and persist throughout the life of the alga. When the fertilized egg (zygote) settles on substrata, one side will produce outgrowths of hyphae that eventually become the holdfast, and the other side will grow into the mature thallus. This growth orientation is very sensitive and has therefore been used to study effects of toxins on development. The apico-basal polarity is most directly determined by light: the side away from the light will become the holdfast. Temperature, acidity, and the growth regulator auxin have also been demonstrated to influence growth orientation.
Prior to the presence of an apical cell, cell division occurs throughout the thallus of the fucoid embryo. When the embryonic plant has reached a height of about five or six millimeters, the apical cell becomes apparent and sinks into its apical depression; this apical cell will be responsible for all subsequent growth.
The primary shoot is formed by the initial diffuse cell division of the embryonic plant. The active division of the apical cell, however, gives rise to a secondary shoot, which will become the main axis of the adult plant. We see here that the base of Cystoseira's axis is then a sympodium. All subsequent branching, however, will follow the monopodial pattern.