226 DR J. M. MACFARIANE ON THE 



/. Geum rivaie. consists of two to three rings of large decayed brown cells ; the layer inside, which 



-'. Geum inter- ° to J 



medium. i s still persistent, consists of rather smaller but neatly quadrangular cells : that more 



■t. Geumurbanum. r . . . ... 



internal is paler in colour, smaller in its cells, but similar in shape to the last ; the inner- 

 most layer consists of small transparent quadrangular cells developing from the cork 

 cambium. Mr Percy Nicol has accurately illustrated this in Plate V, fig. la, In 3 (fig. 1 c) 

 all the cells of the cork layer are very nearly of the same size, no matter what their age 

 may be, and they are 1^ to 2 times as broad as deep on transverse section. In 2 there 

 is an evident radial enlargement of the cells as in 1, but the shape of the cells takes 

 more after 3, though rows of cells may be nearly quadrangular. 



The cortex is similar in the three, except that its cell walls are slightly collenchymatous 

 in 1, very strongly so in 3, and most nearly like the latter in 3. The phloem does not 

 call for description. 



The xylem of 1 is made up of three to five radiating spokes, which are at first separated 

 from each other by the intervention of broad medullary rays. Earely, owing to a slight for- 

 mation of interfascicular cambium, secondary xylem patches appear. Each spoke consists 

 of xylem cells, spiral tracheids, and many large pitted vasa,the largest measuring 40 /x, across. 

 As secondary growth proceeds, interfascicular xylem is laid down, which narrows the 

 medullary rays to one or two lines of cells. This secondary growth is of spindle-shaped cells 

 and pitted vasa. The xylem of 3 consists primarily, as in the last, of three to five (commonly 

 four) radiating masses which are made up of elements like those of 1 during the first year 

 or two, except that the diameter of the largest vas is 24 /*. In time, however (the precise 

 period has not yet been ascertained, but from the position of the roots on the rhizome, as 

 well as their size, I should judge them to be from five to eight yearsold),a ring of dense pitted 

 fibroid tracheids mixed with a few small vasa, is laid down outside the softer and earlier- 

 formed xylem. It may fairly be suggested as a hypothesis to explain this striking differ- 

 ence in the two species that, since G. rivaie grows in moist, damp, shaded, and often 

 sheltered situations, it does not need a special strengthening sheath in its roots to resist 

 pulling strains ; while G. urbanum, growing often in the open on dry, exposed, and wind- 

 swept ground, requires such a sheath for mechanical resistance. In the hybrid the xylem 

 decidedly inclines to 1, though the pitted vasa are from 32 to 36 ft in diameter. The 

 very characteristic thickened zone of 3 is at most represented by occasional isolated patches 

 of fibroid tracheids, which never, so far as I have seen, attain a great size or fuse into a 

 ring. We have here a hybrid tissue taking strongly after one parent, and like that parent 

 the hybrid almost always grows in sheltered places. 



Stem. — The rhizome and flowering stem are very variable in parents and hybrid alike, 

 and will require more detailed attention than I have yet been able to give. The following, 

 however, are broad, constant features in the flowering stem. That of 1 has a scleren- 

 chyma sheath of soft open tissue, has tracheids and vasa in size like those of the root, and 

 large fibroid tracheids. In 3 the sclerenchyma sheath is composed of strongly 

 indurated elements, has small tracheids and vasa like those of the root, along with small 

 indurated tracheids. The hybrid while intermediate inclines rather to 3, and this 



