BY JOHN MCLUCKIE. 299 



of the cells contain a mycorhizie fungus; the infected cells occur in groups 

 (Text-figs. 2, 3) throughout the entire zone from the exodermis to the endodermis. 

 This zone is the cortex of the root; its cells contain much more protoplasm 

 than the sheath cells, but it is mainly confined to the periphery of the cells. 

 Thin cytoplasmic threads extend through the cell-cavity. There is a considerable 

 amount of cell-sap and the cells are normally very turgid. The hyphae in some 

 of the cells are arranged in a loose tangle throughout the cell; in othei-s in a 

 dense mass, staining deeply, around the nucleus or in contact with it. In such 

 cells the majority of the hyphae appear to lose their individuality and mass 

 together, while a few loose ones strike out towards adjacent cells (Text-figs. 4, 

 5, 6). It would appear that one infected cell leads to the rapid infection of 

 neighbouring cells. As a general rule only the cells on the one side of the 

 root are infected, wliile those on the other half may be entirely (or almost so) 

 free from the fungus (Text-fig. 1). 



In some cells the identity of the fungal hyphae is completely lost, and 

 there remains in the centre of the cell a dense, deeply staining, irregular mass 

 in which all trace of hyphal structure has disappeared (Text-figs. 20, 25). The 

 protoplasm and nucleus of the host-cells are still alive; the nucleus appears 

 greatly enlarged (Text-figs. 21, 22). 



No starch is found in cells infected with hyphae. It disappears soon after 

 the infecting mycorhiza enters, but reappears in the host cells when the fungus 

 becomes disorganised. Starch is abundant in all uninfected cells. It occurs in 

 the form of small spherical gTains and stains pink with iodine.* The hyphae 

 stain much more deeply just prior to disorganisation than on entry into a host 

 cell, and appear to contain much protoplasm of a fine gi-anular nature. In 

 newly-infected cells the hyphae are very slender, but their thickness increases 

 somewhat after they have been associated with the host cell for a time. The 

 protoplasmic content of such hyphae increases; it stains more deeply, and 

 becomes dense and granular; the vacuoles which are frequent and relatively 

 large in hyphae which Have just entered cells, become small and few (Text-figs. 

 8, 9). 



The innermost cortical cells are smaller than the central series, and in 

 contrast with infected cells contain abundant starch grains. 



The endodermis consist,^ of a single layer of small cells, containing little 

 protoiDlasm, and having thin suberised walls. Passage-cells occur generally in 

 groups of two or three opposite the xylem groups; the endodermal cells opposite 

 the phloem are thickened. The cells are elongated longitudinally. In trans- 

 yerse section they are almost isodiametric. 



The stele is very small compared with the remainder of the root. There is 

 a single-layered parenchymatous pericycle surrounding 15 or 16 groups of 

 phloem and the same number of xylem gi-oups. Conjunctive parenchyma occurs 

 between the phloem and xylem groups stretching inwards towards the pith. The 

 phloem is composed of very small elements, namely, thin-walled sieve tubes and 

 companion cells. The xylem consists of a few protosylem vessels (annular and 

 spiral) and two or three small vessels with reticulate-scalarifoi-m thickening. 



There is a fairly large pith composed of large thin-waUed cells with sparse 

 protoplasmic contents and distinct intercellular spaces. No mycorhiza. is pre- 

 sent in the pith-cells, in fact the fungus is eonfibed to the cortex and sheath 

 of the root. Abundant starch occurs in the pith-cells. 



*This reaction of starch to iodine in Dipodiiim punctatum is fairly typical of 

 saprophytic Phanerogams. 



