THE HIGHER CRYPTOGAMIA. 415 



it is undeniably favoured by such contact. I have known 

 flowers of female specimens of Juniperus sibirica, which 

 had been completely separated from male ones, to produce 

 normal fruit, and to develope endosperm with fully formed 

 corpuscula. Female flowers of young trees of Pinus cana- 

 densis, which were three miles distant in a straight line 

 from the nearest other trees of the same species, produced, 

 for two years successively, only female flowers, but no male 

 ones. Microscopical examination showed that in both cases 

 no pollen-grains were present upon the nucleus. In both, 

 the formation of embryos was entirely suppressed. 



After the interval which occurs in the growth of the 

 pollen-tubes — and which takes place even in those Coniferae 

 whose seeds ripen the first year — the latter begin again to 

 penetrate towards the embryo-sac. This happens almost 

 at the time when the differentiation of the corpuscula from 

 the surrounding tissue commences. After the tubes are 

 completely formed they reach the endosperm : in Pinus 

 sylvestris and Juniperus sibirica this happens at the be- 

 ginning of June, in Pinus Strobus and Juniperus communis 

 at the end of June of the second year ; in Abies excelsa and 

 Taxus canadensis in the middle of May, in Taxus baccata 

 at the end of May, and in Pinus canadensis at the end of 

 June, of the first year. The deeper the pollen-grains pene- 

 trate into the nucleus the thicker they become, a pheno- 

 menon which is most manifest in Taxus, and least so in the 

 Abietineae. The growth in thickness of the lower part of 

 the pollen-tube of Taxus is so remarkable, that the organ 

 assumes the form of a conical sac (PI. LXIII, fig. 11 ; 

 PL LXIV, figs. 1, 2). The great transverse increase does 

 not commence until the completion of the longitudinal 

 growth. 



About the time when the pollen-tube reaches the endo- 

 sperm, the very thick, hitherto leathery and tough primary 

 wall of the embryo-sac which encloses the endosperm, is 

 softened at the top. The pollen-tube breaks through this 

 wall, apparently after some resistance, (a constriction or 

 sudden narrowing of the tube is often visible at this place), 

 and reaches the double pairs of cells which cover the tops 

 of the corpuscula. Sometimes it makes its way sideways to 

 the corpusculum, piercing through and destroying the 



