94 Practical Plant Biology. 



Just as the stimulus of gravity tends to keep the branch growing 

 in a vertical line, so the stimulus of light tends to orient it in the 

 direction of the incoming light. But these branches are positively 

 phototropic, that is they tend to direct themselves towards the 

 source of light. No more is known regarding the mechanism of 

 the reaction to the phototropic stimulus than is known regarding 

 that of geotropism. 



The top of the vertical branches is at first rounded off. As the 

 branch increases in length its end swells into a tiny knob. This 

 is at first white but as it grows, it becomes greyish and darkens till 

 it finally appears as a black speck or globule on the top of the 

 branch. Microscopic observations on the development of this 

 globule show that in the early stages, when it begins to swell, a 

 . transverse septum is formed at the end of the vertical branch 

 cutting off the enlarging tip from the rest of the ccenocyte. The 

 protoplasm within the tip has many nuclei and they further in- 

 crease in number by subdivision. At the same time the wall 

 covering the globule grows thick, it becomes pigmented and a fine 

 deposit of crystals of calcium oxalate is formed on its outer surface. 

 The protoplasm within this capsule begins now to cleave into 

 fragments. At first each of these contains several nuclei but 

 cleavage continues until the number of fragments is equal to that 

 of the nuclei, and each thus becomes uninucleated. The frag- 

 ments contract and become ovoid, each secreting a cell-wall 

 round itself. They are immersed in a watery mucilage which fills 

 the capsule. Just when the fragments or spores, as we may now 

 call them, have assumed a cell-wall, the transverse septum, 

 separating the capsule or sporangium from the vertical branch, 

 bulges from below upwards into the sporangium thus allowing the 

 upper part of the supporting branch to invade the sporangium. 

 The supporting branch is called the sporangiophore, the intruding 

 part is called the columella. The intrusion of the columella 

 naturally distends the sporangium and often bursts it, or renders it 

 so tense that it explodes at the slightest disturbance, setting free its 

 spores in the drop of contained mucilage. If these spores find 

 their way to a suitable organic medium they will soon germinate 

 by pushing out hyphae which branch and quickly form a mycelium. 

 Later the vertical branches or sporangiophores appear and produce 

 sporangia which liberate spores once more. So the cycle repeats 

 itself. 



Sexual reproduction also occurs in Mucor, but much less often 

 than the asexual production of spores just described. When the 

 hyphae of two mycelia growing in the same medium come into 



