294 Sykes. — Anatomy and Histology of Macrocystis pyrifera and 
increased there is some difference of opinion. Rosenthal considers that 
its growth is to be wholly ascribed to the slow addition of new elements on 
its periphery by transformation from the inner cortex, the only meriste- 
matic layer being the outer layer of the cortex. He lays great stress on 
the distinct radial rows to be traced, in the young plant, from this outer 
layer right into the centre of the pith. He supposes the outer cortex to be 
continually added to on its outer edge, and transformed slowly into inner 
cortex on its inner edge. The inner cortex differs from the outer in being 
composed of more elongated cells, and it is these which, in their turn, give 
rise to those peculiar elements found only in Macrocystis and A T ereocystis, 
which are usually regarded as true sieve-tubes, and which are found, in all 
except the very young stages, arranged in radial rows at the periphery of 
the pith. 
In a stage slightly older than that in which sieve-tube formation begins, 
when most of the original inner cortex has been already used up in their 
production, the remainder of that tissue presents a rather striking appear- 
ance, and it is to this narrow band that we will return below when discussing 
Will’s theory of secondary growth in Macrocystis . 
The later formed inner cortex is made up of cells much less elongated 
than are those of the earlier layers, and hence there is a more or less sharp 
distinction between the two tissues. No more sieve- tubes are formed after 
a certain stage (probably, according to Rosenthal, when all the original 
inner cortex has been used up), and the whole of the further growth in 
thickness of the medulla is due to the expansion of the already formed 
tissues, and to the ingrowth of cortical hyphae. 
The alternative view, due to Will, is that the production of new sieve- 
tubes may go on indefinitely, and Oliver 1 appears to have accepted his 
theory. Will attributes the formation of these elements, in the young 
plant, to a special ‘ thickening ring * which corresponds both in position 
and appearance with the narrow band of tissue referred to above, and which 
he believes functions as a separate meristem. 
Will states that in the older plant this f thickening ring * has no definite 
boundary, but a gradual transition between inner cortex and sieve-tubes 
is then found. It is not at all clear from his account whether he means 
to imply by this statement that there is only an apparent transition, or 
whether he wishes to draw the inference that actual transition from inner 
cortical cell to sieve-tube is at the moment taking place. One would 
suppose, however, from the general terms of his description, that, unlike 
Rosenthal, he considers growth in thickness, by means of a secondary 
formation of new sieve-tubes, to be a long continued process, and that he 
therefore believes in the production of new young sieve-tubes in the 
old plant. 
1 Oliver, 1. c., 1887. 
