PH £OPHYCEA 79 
growth proceeds they are left behind, as it were, 
by the upward advancing growing-point, and by 
the time they are mature they are situated some 
distance below it, while new sporophylls are formed 
successively above them, imparting a pinnate ap- 
pearance to the frond. In Heklonia, Ulopteryx, and 
Hisenia the sporophylls arise at the base of the blade 
rather than at the tip of the stalk, while in Zgregia 
they spring from the entire length of the growing- 
point, and thus when full-grown are situated on 
the margins of both blade and stalk. 
As the result of the operation of these modes 
of growth acting in diverse ways the Laminariacce: 
present a considerable variety of forms. Chorda, 
as has been mentioned, possesses no leaf-blade, 
but the other genera approaching Laminaria 
itself exhibit great fronds, simple as a rule, but 
also digitately divided as in Laminaria digitata. 
The most humble form of all is Adenocystis, in the 
southern ocean, a small sac-like, ovate frond on a very 
short stalk. The fronds of Agarwm and Thalassio- 
phyllum are perforated, and the latter is branched, 
but the branches do not arise as in Lessonia ; they are 
proliferations from themargins of newly formed perfora- 
tions at the growing-point. There are midribs in the 
fronds of Agarum, Cymathewre, Ulopteryx, and Alaria, 
while Costaria has several ribs each confined to its 
own surface of the frond, three on the one side and two 
on the other. The blade of Dictyoneuron has a network 
of prominent veins, while Postelsia has longitudinally 
wrinkled blades. Nereocystis, Macrocystis, and Egregia 
possess air-floats, those of Nereocystis being of great 
