46 MANUAL OF BOTANY 



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

 long unbranohed 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 unbranohed forms. On the stalk, in some oases, 

 gouidiophylls are developed in addition to the ordinai-y fronds, 

 their development being acropetal. In some of the fronds 

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

 like appearance. A few forms possess air-floats, something like 

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

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

 of rook or stone. 



The thallus shows still more complete histological differentia- 

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

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

 comes merismatio, though in many species the meristem is found 

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

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

 able thickness. Like most merismatio layers, 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 sieve-tubes of which mention has already been 

 made. 



The central strand is something like that of the larger 

 Fucacese, consisting of hypha-like filaments which anastomose 

 copiously. This layer is continued upwards into the fronds. 



The mucus passages abeady alluded to are only found in 

 some of the Laminariacese. 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 cUiated zoogonidia or 

 zoospores. Some of the genera possess cryptostomata, which 

 bear paraphyses as in the Fucacese. 



The genus Splachnidium appears to occupy an intermediate 

 position between Fucacese and Laminariaoeae. It is peculiar in 

 that it bears sporangia or gonidangia resembling those of the 



