92 Practical Plant Biology. 



hyphae is killed and stained, the cytoplasm may be very plainly 

 observed lying in contact with the inside of the cell-wall and 

 forming a film of irregular thickness coating its inner side. Some- 

 times one sees filaments and bridges of cytoplasm traversing the 

 axial vacuole. In the cytoplasm scattered nuclei may be seen. 

 Treatment with iodine brings no starch-grains to light but in their 

 place scattered oil-drops or fat-globules are found in the cyto- 

 plasm. The hyphae of the mycelium taper gradually towards 

 their tips. The latter appear filled with protoplasm. Observation 

 shows that the growth of the mycelium is localised at these tips. 

 The vertical branches have the same structure as the other hyphae 



FIG. 17. Mucor. a, mycelium with sporangia in different stages of de- 

 velopment, x 70 ; 6, conjugation-branches in contact with one another ; 

 , zygospore supporting a sporangium, x 150. 



and are covered with the same continuous cell-wall. The proto- 

 plasm and vacuole are usually continuous throughout the whole 

 plant. 



Although Mucor is not motile like Volvox, the direction of its 

 growth has been shown, like the motion of Volvox, to be de- 

 termined by external stimuli. Thus it has been found that the 

 growing tips of its hyphae are oriented in such a manner that they 

 grow towards the point of greatest concentration of substances use- 

 ful in their nutrition. It may also be inferred from analogy with 

 other plants, that harmful substances-; or harmful concentrations 

 exercise a repellent action on these hyphae. 



