Lecture XIII. 99 



wall is formed cutting off the posterior more vacuolate portion as 

 a separate segment from the growing tip, which continues its for- 

 ward growth. ' Once the segments are thus cut off from the growing 

 apex they cease to elongate markedly. Lateral branches develop 

 as outgrowths from the segments usually immediately behind the 

 transverse wall nearest the apex. These outgrowths push out to 

 form new cylindrical growing segments and by elongation and 

 subdivision develop into new lateral hyphse. By a continuance 

 of this process from many segments of the first hypha complicated 

 branching systems are developed. As the growing tips of all the 

 hyphce respond chemotropically in a positive sense to the concen- 

 tration of food substances in the medium, the whole mycelium 

 spreads in a radial manner, and extends from the region where 

 the food-substances are exhausted by its own absorption to the 

 surrounding regions as yet untapped by its inroads. 



When growth of the mycelium has proceeded for four or 

 five days upright branches are formed, the first appearing in 

 the oldest and consequently the central parts of the mycelium. 

 They resemble the hyphae of the mycelium, so far as can be 

 seen, except in their response to external stimuli, which causes 

 them to assume a vertical instead of a horizontal position. 

 Each vertical branch consists of an unbranched shaft formed 

 of four or five cylindrical segments. The rigidity necessary to 

 retain them in the vertical position is conferred on these branches 

 by the osmotic pressure within the vacuoles of the ccenocytes, 

 as may be seen when the cytoplasm is rendered permeable by 

 iodine vapour or bisulphide of carbon. On the top of these 

 are supported segments which give rise to lateral branches, and 

 the segments of these latter may give rise to more branches. 

 As all the branches formed respond in the same manner towards 

 gravitational force they arrange themselves parallel to one another 

 and vertical to the mycelium. The vertical branching system 

 reminds one of a complicated candelabrum. On the terminal 

 ccenocytes or segments of this branch-system, which is called 

 a conidiophore, a small group of peculiar shaped ccenocytes 

 is formed. Each of these is bottle-shaped, having a cylindrical 

 basal portion continued into a narrow neck-like part. The 

 latter is called the sterigma. The bottle-shaped ccenocytes 

 occupy the position of the candles in the candelabrum. The 

 tip of each sterigma is covered with a plastic wall and the 

 internal pressure in the ccenocyte bulges it out to form a minute 

 globule -a conidium. This is a tiny mass of protoplasm con- 

 taining a nucleus, and soon its covering membrane becomes 



7* 



