A STUDY 01? RECIPROCAL CROSSES. 83 



Just how the pollen tube is nourished, how it is guided to the 

 ovarian region, and what finally brings the elements together, 

 is still very largely a matter of theory and conjecture. However, 

 imperfect as our knowledge is concerning these points, numerous 

 observations on various stages and types of germination, enable 

 one to make a few very general remarks on the above questions. 



As regards the nourishment of the tubes, it is known that there 

 are two probable sources of food. The pollen grains contain 

 some elements for nourishment, and the conductive tissue is not 

 entirely without food. From chemical analyses it has been 

 shown that pollen cells contain a certain amount of nutritive 

 material such as starch and maltose, in connection with some 

 invertase and diastase to render them available for the nutrition 

 of the pollen tubes. On the part of the pistil and its conductive 

 tissue there is an abundant supply of elaborated food materials 

 such as starch, sugar, and maltose, which, without doubt, are of 

 much value in nourishing the tubes in their passage to the 

 ovules. 



It is very probable that the development and action of enzymes 

 facilitate the descent of the pollen tube, and the discovery of a 

 cytolytic enzyme by Marshall Ward 1 in a species of Botrytis, 

 throws further light on the problem of the penetration of tubes. 



An analysis of the factors which facilitate the descent of pollen 

 tubes and finally enable them to reach the ovules shows them to 

 be of at least two sorts. First, mechanical contrivances ; and 

 secondly, stimulating influences or attractive substances which 

 cause the union of the elements. The mechanical features 

 include the nature of the conductive tissue which, as we know, 

 is a system of thin walled cells through which the pollen tubes 

 pass, growing as they naturally would in the lines of least resist- 

 ance. Other features of a mechanical nature are also useful ; 

 such as papillae, which are common in the pistils of Cucurbit- 

 aceae ; and the position of the funiculus and the ovule itself. 



The second set of factors, the stimulating or attractive sub- 

 stances, do not admit of as easy demonstration. As a matter of 

 fact, just what actually brings the male and famale nuclei into 

 contact is not known. However, a few probable suppositions 

 may be mentioned here and left for further discussion and 

 investigation. 



1. H. Marshall Ward, Annals of Botany, II, 188S. 



