tJic Marine AJrjce of the Firth of Forth. 433 



on ail allied genus, to the effect that the zoogonidia of 

 Ulva are positively heliotropic, whilst the conjugated 

 zoospores have an opposite or heliophohic tendency. From 

 the fact that, in the glass vessels where (except for 

 the slight disturbing influence exerted by the aerating 

 apparatus) the swarmspores were allowed to take up any 

 position peculiar to them, they ultimately were found in 

 greatest abundance on the side directed towards the 

 greatest amount of light, where, however, the direct rays 

 of the sun were never allowed to impinge, as well as 

 from their behaviour in the field of the microscope, I am 

 inclined to regard the existence of positive heliotropism, 

 but on the shaded side of the vessels young embryos were 

 also by no means wanting. 



This species of Enter omorpha was the earliest and most 

 tenacious occupant of the wooden surfaces, and the greatest 

 number of observations were made on it in its earliest 

 stages. I was, however, able by prosecuting the same 

 methods of observation to collect many samples of Por- 

 'phyra vulgaris in a ripe or spore-bearing condition, and to 

 watch the emission of the swarmspores. These, however, 

 did not germinate in captivity, nor were even the first 

 stages of their development otherwise observed. The 

 swarmspores of this species, however, moved more slowly 

 and apparently more regularly than those of the Entero- 

 moyyha, and the movement was continued for a somewhat 

 greater length of time. 



As has already been pointed out by Rosanoff,t in the 

 case of the Ulvaceoi generally, specimens of Enteromorpha 

 compressa,Monostroma latissirnum, Ulva /m~a,&c., from which 

 the spores have been emitted, exhibit local changes of 

 colour at the points where emission has occurred — changes 

 which, however, cannot be regarded as primarily patho- 

 logical, although subsequent circumstances now more readily 

 bring about the rupture of the thallus at the weakened areas 

 and sympathetic abnormal conditions in the adjoining cells. 



The following table is given for the purpose of further 

 showing the results of a series of measurements which 

 have from time to time been made of Alga3, which have 



t " Observations sur les functions et les proprietus dcs Pigments dc deverses 

 Algiies," &c., Mem. Soc. Imp. d. Sci. Nat. Clicrlourcj, t. xiii. 1867. 



