170 GROWTH OF POLLEN -TUBE [CH. 



In other cases (e.g. Grasses) the tube penetrates the 

 middle lamella between the cells of the feathery stigmas, 

 exactly as many fungus-hyphaa do. 



In other cases (e.g. Malvaceae and Caryophyllaceae) the 

 parasitic action of the pollen-tube is still more marked, 

 for it penetrates the cell-walls and passes across from one 

 cell-cavity to another just as do hyphse which perforate 

 the cells of their host. 



Nor are these the only respects in which pollen- 

 tubes resemble parasitic fungi. Enzymes have been 

 found in them, quite like the enzymes secreted by 

 hyphae to dissolve their way through the tissues, and 

 food-materials such as starches and sugars are digested 

 by the protoplasms of the tube. It has also long been 

 known that the protoplasm is kept aggregated at the 

 apex of the tube, as in growing hypha?. In some cases 

 the tube forms curious little cellulose blocks, or stoppers, 

 behind this apical protoplasm (e.g. Orchids) as if to 

 prevent all danger of back-flow. 



In Conifers also, where the pollen-tube passes directly 

 into the micropyle and nucellus, and lives for months in 

 the tissues of the latter, one can see the cells in the 

 neighbourhood of the passage bored out by the pollen- 

 tube killed and turned brown as if corroded. 



Even apart from these direct proofs of the nutrition 

 and growth of the pollen-tube, we should be able to infer 

 their occurrence from the enormous lengths to which the 

 tubes grow in long-styled plants (e.g. Crocus), and the 

 time, weeks and even months, during which they live in 

 the tissues (e.g. Pines, &c.). Moreover, the rapid and far- 

 reaching alterations, induced in numerous cases in organs 

 far removed from the seat of action of the tubes while 

 still in the style, suggest analogies at once with the 

 similar stimulations provoked by parasitic hyphse in 



