KUNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 55. N:0 5. 6 



stratified cellulosa layers (= thickwalled o-cells), and finally cells with unilateral in- 

 crassation (= w-cells). Yet the degree of incrassation very much depends on the 

 age of the plant, wherefore it is most clearly observed in fruiting individuals. Be- 

 sides there are several varieties of the latter two celltypes, some of them due to 

 alteration of the form of the cell-room, which varies from broadly rounded to nar- 

 rowly oval, especially in the w-cells. P. luce?is offers example of the first-mentioned 

 broad w-cells. while the P. gramineus has the compressed form of those cells. 



Species are met with having the w-endodermis not fully developed, but with 

 a tendency to form w-cells. Such an interesting species, for instance, is P. po- 

 lygonijolius and several others. These species have a pure o-endodermis at the nodes. 

 But at a very little distance from the node the onesided incrassation gradually 

 comes in and always in the corners of the central axis outside the vascular bundles, 

 where also the supply is by far more necessary than in the sinus close by, in which 

 on the contrary thinwalled communicating cells are required. The evolution conse- 

 quently shows that species with an o-endodermis are more primitive than those 

 provided with a ?<-endodermis, and that the anatomical conditions of the stem in 

 their more advanced state are to be found towards the middle of an internode, 

 whereas the more primaeval state may be studied in the very node. 



P. Robbinsii furnishes another example of a mixed endodermis, which we will 

 consider when in the dissertation we arrive at this species. It is composed by both 

 thinwalled and stratified cells. — On taking a review of the genus you accordingly 

 will find five (or six) essentially different kinds of endodermises. 



A. Uniform endodermises: 



1. Non-stratified o-endodermis. 



2. Stratified o-endodermis. 



3. C/-endodermis, with two modifications: 



common w-type, cells with round or broadly rounded cross-cut form, and 

 gramineus-ty^e with laterally compressed cells and narrower lumen. 



B. Mixed endodermises: 



4. — 17-endodennis : a u-enå. with o-cells at intervals or faintly developed w-cells. 



5. 0— O-endodermis, with o-cells of different kinds. 



The endodermis encloses the central axis, which principally consists of pith, 

 vascular bundles chiefly belonging to the leaves and often also a strengthening tissue. 

 These mechanical cells most frequently follow along the endodermis as a more ore 

 less complete layer and sometimes form moreover an intermediate stratum between 

 the two median bundles as well as between the lateral bundles and the median ones, 

 by which a fusion of bundles is prevented. Finally they appear as more or less 

 strong open sheaths round the vascular bundles, thus constituting a sclerenchymatous 

 stele. Non-sclerenchymatous species are more or less fully destitute of particular 

 mechanical cells in the pith. 



The topography of the stele chiefly depends on the arrangement of the bundles. 

 These namely behave in some species or groups in a dissimilar way to what is the 

 fact in others. A fusion of bundles frequently takes place according to a certain 



