232 



DR J. M. MACFARLANE ON THE 



/. Saxifraga 



Geum. 

 ?. Saxifraga 



Andrewsii. 

 I. Saxifraga 



Aizoon. 



informs me that he has failed to find fruit on it. The form, however, is one which yields 

 valuable results from the standpoint of hybrid histology. 



(e) Saxifraga Andrewsii, x . 



This plant has given rise to some discussion, having been regarded by certain botanists 

 as a true species, though by most as a hybrid. It was first brought under the notice of 

 botanists and cultivators by the late Mr Andrews, who asserted that he had gathered it in 

 a wild state at the head of Glen Carragh, Co. Kerry. Most botanical experts to whom 

 the plant was submitted viewed it as a hybrid between S. Aizoon and S. Geum, or some 

 nearly related species, and that it had probably originated in Mr Andrews' garden, and 

 was confounded by him with some finds from Kerry. It is on this ground well suited as a 

 plant to test the accuracy of the present inquiry. My earlier examination soon convinced 

 me of its hybrid nature, as well as its relation to S. Geum on the one hand, and a 

 crusted species on the other ; but on the " Aizoon " side I had to work very carefully 

 over, and compare with each other such nearly related species as S. nitida, S. mutata, 

 S. Hostii, &c. Several of these are probably rightly regarded as sub-species or varieties 

 of S. Aizoon, though I found them constantly to differ in several fine structural details. 

 S. Aizoon entirely satisfied the histological requirements of the other parent, and we 

 shall now see how the hybrid blends the features of both. 



Stem. — The structure of this is considerably complicated by the leaf traces which are 

 constantly passing out to the crowded leaves, and also by similar root traces. I may, 

 therefore, begin with the pith tissue, which is large in amount in 1, and surrounded by a 

 ring of bundles. The pith-cells are pretty uniform and large, 50 to 60 /x in diameter, 

 and starch granules are abundantly stored up in them. These granules vary greatly in 

 size, but the largest are 6 to 7 /u, across. In 3 the pith has a triarch or tetrarch outline 

 from its relation to the surrounding bundles ; it is small in amount, its cells are of very 

 varying size, the largest being 35 \i across. The largest starch granules are 2 /x across. 

 The hybrid rather approaches to S. Geum in having the pith and pith-cells large, though 

 not equal to the parent, while the largest starch granules are 4 fi across. Outside the 

 pith is a colloid layer traversed by canals, and this is largely developed in S. Geum, 

 narrow in S. Aizoon, and intermediate in the hybrid. 



Leaf. — The colour and shape of the hybrid leaf are between those of the parents. A 

 special difference in S. Aizoon and all the crusted Saxifrages, as compared with S. Geum 

 and its section, is in the excretion from the former of lime salts by the water stomata 

 situated at the tips of the serrations on the upper leaf surface. These salts on precipita- 

 tion form a crust-like mass over each stomatic area, and give a variegated appearance to 

 the leaf. In the " Geum " section salts never precipitate. Though the hybrid often gives 

 little indication of a lime crust when grown in one position, or under one set of condi- 

 tions, I have repeatedly got specimens in which it was very pronounced to the eye, and 

 gave characteristic efflorescence when acted on by acids. 





