PH.EOPIIYCE.E 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 Ecldonia, Ulopteryx, and 

 Eisenia the sporophylls arise at the base of the blade 

 rather than at the tip of the stalk, while in Egregia 

 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 Laminariacecc 

 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 Agarum and Thalassio- 

 phyllum are perforated, and the latter is branched, 

 but the branches do not arise as in Lcssonia ; they are 

 proliferations from the margins of newly formed perfora- 

 tions at the growing-point. There are midribs in the 

 fronds of Agarum, Cymathwre, 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 Egregici 

 possess air-floats, those of Nereocystis being of great 



