650 SUMMARY OF CURRENT RESEARCHES RELATING TO 



formed in the conducting-tissue of the stigma are moderately uniform. 

 Pollen-grains of Leucojum cestivum immersed in the same way at 

 7 a.m. began to put out their tubes at 10 .30 ; about 11 .30 the tubes 

 were twice as long as the grains; about 12.0 three times as long, 

 and at 2 p.m. ten times as long as the grains ; increasing in length 

 about 0-1 mm. per hour. If the mucilage is too watery, the grains 

 are liable to burst. Tinting with carmine-ammonia exhibits the 

 development of the pollen-tubes very well when the mucilage in which 

 they are immersed is sufficiently fresh. 



Cause of the Movement of Pollen-tubes.* — The penetration of 

 pollen-tubes into the conducting tissue of the style is attributed by 

 Sachs to unequal growth of the two sides ; A. Tomaschek, on the 

 contrary, considers it to be due to hydrotropism. When masses of 

 pollen of Colchicum autumnale are placed in a hollow plum from 

 which the stone has been removed, the pollen- tubes from the upper- 

 most grains rise erect, while those which lie at the side incline 

 downwards. Pollen-grains made to germinate in the open air display 

 curvatures and even spiral windings, closely analogous to those of 

 tendrils, which can scarcely be assigned to any other cause than 

 revolving nutation. 



Apical Growth of the Roots of Phanerogams.^ — S. Schwendener 

 gives the following summary of results of his own and of previous 

 investigators on this point : — 



1. Most dicotyledons have a formative tissue over the apex of the 

 root, the innermost layer of which is the young epidermis, the remain- 

 ing layers belong to the root-cap. The originally simple row of 

 cells of which the epidermis is composed splits into two ; the inner, 

 and sometimes the outer one of these again divide. In a small 

 number of dicotyledons layers of cells split off from the epidermis 

 outwards, constituting the root-cap ; the root-cap and the root itself 

 having in these cases a common histogen. The formation of the 

 root-cap is shared by the epidermis, which covers the apex of the root 

 either in an undivided or in a split condition, and by the entire 

 cortex or its outermost portion only. Monocotyledons have distinct 

 histogens for the root-cap and the root itself; the epidermis is not 

 in genetic connection with the root-cap ; but there is a common 

 primary meristem for both. Dicotyledons and monocotyledons 

 present therefore this difference ; that in the former there is a 

 genetic connection between the root-cap and epidermis, but not in 

 the latter. 



2. The author confirms the statement of Eriksson that in dicoty- 

 ledons the root-cap and epidermis proceed from a common histogen, 

 the " dermacalyptrogen " ; although differentiated dermatogen cells 

 take part in the formation of the cap. 



3. There is no distinct histogen, as Hanstein asserts, for the 

 vascular cylinder or plerome. 



* SB. K. Akad. Wiss. Wien, lxxxiv. (1881) pp. 612-5 (1 pi.). See Bot. 

 Centralbl., xi. (1882; p. 12. Cf. this Journal, ante, p. 372. 



t SB. K. Preuss. Akad. Wiss., 1882, pp. 123-39 (2 pis.). See Bot, Centralbl., 

 x. (1882) p. 389. 



