432 



tresses of strong stereom occur above and below the largest 

 bundles; but in the case of the smaller bundles only below 

 them; the smallest are entirely without stereom. There is in 

 these leaves, upon the lower side of the sympodial marginal 

 vein, an especially strong fibrous tissue extending to the epi- 

 dermis, which is not the case with the other bundles. 



The young partially expanded leaf is almost isobilateral 

 in structure, and is without intercellular spaces of any size, 

 usually two palisade-layers are gradually differentiated and may 

 constitute as much as one-half of the mesophyll, the cells occur 

 close together and are about twice as long as they are thick. 

 Large intercellular spaces are also gradually developed in the 

 spongy parenchyma, and, as shown in the figure, are separated 

 by vertical lamellae. The transverse and the longitudinal sections 

 are similar in this point. 



No true epithema occurs, but the vein extends almost 

 to the epidermis of the tooth of the leaf-apex, and upon the 

 upper side of this tooth 2—3 stomata occur with the same 

 appearance as that of the water-stomata in B. nivalis: stomata 

 are absent from the under side of this tooth. 



The leaves are rich in large, yellow sphæro-crystals (Fig. 

 oS, F) , especially abundant along the veins and under the 

 epidermis; they are probably of substances which have been in 

 solution in the living cell, but were precipitated by alcohol ; 

 their nature has not been more closely investigated. 



The leaf- s talk, apart from its form, is similar in structure 

 to the peduncle. 



Summary. 



I. The growth -form of the species which have been 

 investigated may be referred to the following groups. 



A. The Primula-type. The species referred to this group 

 have a vertical, perennial rhizome, the primary root probably 



