46 MANUAL OF BOTANY 



Phaeophyceae ; they show a good deal of variety of form, including 

 long unbranched fronds terminating below in a thick stalk, as 

 well as stems which bear a series of much and variously divided 

 leaves. In some the stalk is so short and covered by rootlets, 

 while the fronds are so long, that a single plant appears like a 

 group of the unbranched forms. On the stalk, in some cases, 

 gonidiophylls are developed in addition to the ordinary fronds, 

 their development being acropetal. In some of the fronds 

 midribs and subordinate veins appear, giving them a very leaf- 

 like ai)pearance. A few forms possess air-floats, something like 

 those of the Fucaceae. The stem or stalk terminates below in 

 strong rootlets or haptera, which fasten the plant to a substratum 

 of rock or stone. 



The thallus shows still more complete histological differentia- 

 tion than that of the Fucaceae. The stalk shows epidermal, 

 cortical, and medullary tissue. The epidermal layer often be- 

 comes merismatic, though in many species the meristem is found 

 in the peripheral part of the cortex. This merismatic layer is 

 often very active and causes the stalk to become of consider- 

 able thickness. Like most merismatic laj^ers, it forms new 

 tissue on both its faces. The inner portion of the cortex 

 in the stalk consists of a layer or cylinder of elongated cells 

 whose walls are generally pitted. In Macrocystis it contains 

 the peculiar sieA'e-tubes of which mention has already been 

 made. 



The central strand is something like that of the larger 

 Fucaceae, consisting of hypha-like filaments which anastomose 

 copiously. This laj^er is continued upwards mto the fronds. 



The mucus passages already alluded to are only found in 

 some of the Laminariaceae. They are long tubes which branch 

 and anastomose, forming a network in the tissue. 



The growing point is sometimes apical, sometimes inter- 

 calary. 



The reproductive organs are not produced in conceptacles 

 but in sori, variously distributed over the fronds, but sometimes 

 confined to definite sporophylls. Each sorus consists of a 

 number of unilocular gonidangia, among which are found 

 paraphyses. The gonidangia give rise to ciliated zoogonidia or 

 zoospores. Some of the genera possess cryptostomata, which 

 bear paraphyses as in the Fucaceae. 



The genus Splachnidium appears to occupy an mtermediate 

 position between Fucaceae and Laminariaceae. It is peculiar in 

 that it bears sporangia or gonidangia resembling those of the 



